I looking at adding a dampening factor adjustment to one of my SS amps. I'm looking at loosening up the feel a bit and before I put it under the soldering iron I thought I would ask around here.. My first impression would be to add more series resistance in the neg feedback loop at R108 any other thoughts?
"Normal" voltage feedback makes amps with high damping and tight, dry bass.
"Current" feedback amps do the opposite.
Yours has (like most modern SS guitar amps) a mixture of both, set to a point the designer liked.
To have a taste of the different flavors, you can add a couple switches which do the following
1) Shorting R110 will kill current feedback, increasing damping.
2) Opening R109 (lifting one end from ground) will increase current feedback, decreasing damping.
Good luck.
Thanks for the explanation :) I'm used to tube amp power stages. I didn't know current amps behave in the opposite. I think I'll add some switches and see what happens..
I noticed that effect on my own a ***long*** time ago.
I started making guitar amps in 1969 , all tube of course.
In 1972 Argentina defaulted on its external Debt (what Greece is about to do now) and, not having U$$ available, imports were impossible.
Tubes dissappeared from the shops, just like that .... or were worth their weight in gold.
Started using Transistors, which were still affordable, but noticed that the sound was not the same.
Part of it was that SS amps had too high damping (approx. 100 ) which caused "dry" bass; I measured my Twin Reverb type tube amps, and damping was around 1.
I added current feedback (straight from SS design books) until I got the same value.
It helped a lot.
I guess I invented "Valvestate" on my own, about 15 years earlier than Marshall, go figure.
Necessity is the Mother of Invention.
I finally got back to that amp. We lost power here for a week. I put 1 meg audio taper pot in series with R109, routing the wiring through the low input jack. I'll try it out at my next rehearsal and report back. I have another amp with a different topology I want to try this with next so I may have to pick your brains some more. Thanks for the help, J M
The pot worked but I think I'll put a much smaller value one in and mount it on the back. After about 10k of resistance there isn't any change in feel. My other amp a PV Special has a discrete transistor output driver stage. Looking at the schematic I think raising R79 would increase the current feedback. R96 is the current sensing resistor? There dosen't seem to be any voltage feedback? Does that look right to you guys?
Voltage feedback is R77/R78 and R79 in parallel.
Varying R79 will have a *slight* effect, not much.
Current feedback is Zspeaker/R96.
Both voltages are added because R78 is in series with R96.
The switching I suggested earlier worked by killing either voltage or current feedback, leaving "the other" alone, and has appreciable effect.
Adding pots works, in theory, but real world, store available values make adjustments subtle.
The standard Peavey circuit shown gives a voltage gain without speaker of ~41 but with a nominal 4 ohm speaker this varies approximately between 19.2 and 36 because speaker impedance is greatest at resonance (typically 38-40 ohms at ~110Hz with a closed back amplifier) and also increases from the nominal value as frequency increases from about 600Hz. Now damping characteristics of the amp also change with frequency and speaker impedance. At 4 ohms speaker impedance damping factor will be 0.9 and amplifier output impedance 4.5 ohms (a reasonably good match) while at speaker resonance, i.e. 40 ohms the amplifier output impedance will only rise to 5.5 ohms and damping factor will increase to about 7.3, which is a good thing as it stops the speaker flapping about at resonance.
Look at Rod Elliot's stuff from ESP, Australia for more info on how to calculate all this. I've set up a spreadsheet to convert an Award Session Rockette 30 and am getting quite good results for a 'tube type' sound.
Hope this is useful.
Steve Mitchell
Quote from: J M Fahey
I guess I invented "Valvestate" on my own, about 15 years earlier than Marshall, go figure.
No JM, you invented it on your own, but Marshall ripped it off.
Attached is from "High Fidelity Sound Reproduction", 1958, Newnes. I tried this on a KT88 bass amp build in about 1968 but, not understanding that the Celestion 18 was already in a grossly overdamped cab, found it didn't make any difference. I've only started to reconsider this idea since reading Rod Elliot's stuff and thinking about trying to make SS amps sound more valve-like.
It's cruel that nobody gets credit for nutting out something second, but I have no doubt at all that Jim Marshall would have been very well aware of the prior RCA work.
The utilization of current feedback - first in positive format - to alter output impedance and damping characteristics dates back to about early 1950's and tube technology. At first it was used to straighten out the amplifier's response. There were several articles about the invention in various magazines of the art and eventually the design was even patented by a certain Bogen designer who was also responsible for writing various magazine articles about the idea.
Aside a few applications the idea never caught much fire in a large scale. From that point on the scheme was used now and then, in guitar amps at least Seymour Duncan utilised it. They pretty much carbon copied a certain magazine circuit.
Current feedback in negative phase form, to "unstraighten" the response of a solid-state amp, was introduced already in 1965 in Triumph's solid-state amps. That's at least the first reference I've seen of it so far. From then on various manufacturers have claimed they have "invented" it and that it is something entirely new, groundbreaking technology. They usually have their own market-appealing pseudoscientific name for it - likely because the guys who market these amps realize it's more wortwhile to give an impression that the amplifiers feature some super exciting proprietary feature than to simply state that their amplifiers are copying the exact same idea as 100 other amp makers.
A short list of where I've "first" seen negative current feedback used to mimick tube amps is somewhat along these lines:
- Triumph amps (1965)
- Ampeg SST/SBT series (1969)
- Jordan 120 & 140 (1970)
- Polytone amps (1975)
- Rickenbacker TR series (1977)
- Randall amps (ca. 1977)
- Risson amps (ca. 1978)
- Barcus-Berry amps (1979)
- Dynacord GS-series (1980)
- Peavey (ca. 1980, possibly earlier)
- Fender "CIP" (1981)
- The famous Carver's "Challenge" to HiFi magazines and following series of amplifiers (1985)
- Crate G-series amps (ca. 1986, possibly earlier)
- Rickenbacker RG-series (1989)
- Rocktron (1991)
- Marshall Valvestate series (1991)
...so at the point "Valvestate" amps appeared the technology was already well-established and pretty much a standard feature in guitar amps. If I remember right, someone tipped me of a german book about transistor guitar amps written in early 1980's and the book already discussed the principle. The patent of Dynacord's tube emulation from 1980 just briefly mentions the featured current feedback, and basically makes no big deal about it since at the point everyone making guitar amps already seemed to use the scheme. And like J M Fahey states, it can even be discussed in good analog electronics reference books. So, wake me up if you can actually point a reference prior 1965 of "inventing" this thing.
Did Marshall "invent" it? Well, obviously not. Were they aware of that type of circuit? Most definitely, those guys ain't stupid. Would their amps have sold like hot cakes without the "Valvestate" hype...? Hell No! Like said, those guys ain't stupid.
I did research this trick long time back using discrete's on breadboard.
Tested some of ESP ideas along with others but I never found it did much.
But hey I'm only the hobby geek here so I may have missed the point :lmao:
My point being maybe it only works well when big Amps are cranked up through a stack of speakers.
Through small combo rigs of say 30 /50 watts it's hardly worth the effort.
A local chap has a shed full of Peavey Amps and I've had the chance to hear quite a few Early and later Bandit type rigs.
And None with the fancy FB have seriously impressed me.
Phil.
I think it really falls down to how each individual perceives certain characteristics in tone.
Personally, I hear the effect only as slight boost at low and high frequencies. I usually have to struggle to hear it as well.
Then again, I've encountered people who say it makes an astounding difference. Last one saying so wasn't even talking about current feedback per se but about differences of running a 100% tube amp to either purely resistive dummy load, or to a dummy load that mimicked speaker impedance. The effect / difference is essentially the same.
For him the resistive dummy load was too sterile, the reactive was "squishy" and responding to playing dynamics making a "Night/Day" difference. Personally, I had to struggle to hear the slight difference. ...as usual.
I once angried someone by stating that only thing the "Reactance" control in his Rocktron Velocity did was basically equivalent to diming bass and treble controls of a generic HiFi -style EQ - nothing else. He got mad insisting the control turned his amp to touch-responsive dynamic setup that sagged like a real tube amp. ...all that from a generic boost of low and high frequencies. The control didn't even try to mimic the unique response of a poorly damped amp driving a loudspeaker. It just introduced a basic treble and bass boost.
It's all in how you perceive things, and perceiving can be based 99% on imagination and 1% on "real" auditory information. It's always more or less subjective. Therefore I wouldn't jump to definite conclusions too quickly. Yes, objectively viewed the damping barely has a slight effect on frequency response... but so far I never encountered anyone who would sense things 100% "objectively".
Weeelll, it's easy to do "research" today, specially thanks to Google, or Internet in particular. ;)
No such thing in 1972 !!!
Nor widespread sharing of schematics either.
I got my Marshall, Hohner, Farfisa, Fender , etc. schematics, by drawing them with pencil and paper, out of an open chassis on my bench.
No other way.
Not forgetting that up to that time *most* amps were still tubed.
And no widespread "buildable" 100W amps schematics either.
The 2 most powerful ones which *were* available (schematics "on paper" using commercially unavailable transistors such as Delco's DTG110 do not count) were the "RCA 70W" with 40411 output transistors and Philips/Fapesa/Miniwatt transformer driven 100W amps with 2 x 2N3055H outputs, driven by a 3rd 2N3055, class A.
Later they offered a 200W schematic of a bridged amp, using 2 silicon transistors and 2 germanium, go figure.
And no short protection either.
All with voltage feedback, of course.
Triumph??? Jordan??? Maybe .... but *WHO* knew them?
Even today !!!!
Never seen any of their schematics, only "Jordan BossTone" which is a pedal.
I was lucky to have one copy of Jack Darr's great book, probably the 1968 edition, with 3 Ampegs: BT15, the 5 channel PA100 and a crazy ST something guitar amp.
All 3 shared the same Power Amp, with 4 40411 output transistors, 80V single supply (so needed output capacitors).
No output short protection !!!!
They relied on modified XLR connectors to avoid shorts (which are inevitable with standard plugs) and one of them had a *lamp bulb* or a ¿4 ohm 50W? resistor in series with the extension speaker jack. Go figure.
And, of course, *none* of them used current feedback!!
Nor anybody else which was widely known, such as Peavey. ;)
Besides, as I said earlier, schematics were *not* available or circulated to the general public.
Years later I was the Technical Consultant for Import Music, which among many other brands, represented Acoustic, and I got a huge folder full of schematics, many of which unreadable photocopies of photocopies of .....
So I stick to my guns about having independently invented and applied mixed feedback technology in 1972.
All others mentioned (besides the 3 first: 2 unknowns and one which did not visibly use it) came later, as per the amp list posted above.
In fact, I did not take the idea from any amplifier, guitar or otherwise, but from reading about regulated (discrete) power supplies, which was a widely discussed subject.
Most were constant voltage (of course, that's the point); a few were "exotic" (by 1972 standards) current sources, meaning current feedback.
I understood and could design feedback very well (was doing 4th year Engineering by then) and had the idea to make an amp with 4 ohms internal impedance, something I had *measured* in the Twin clones I was building .
Since voltage feedback made a PSU (and an Amp) behave as having zero impedance, and current feedback made it behave as having infinite impedance, surely a combination of them should behave as having a definite intermediate number !!!
In this case the desired 4 ohms.
With those parameters in mind , it was easy to calculate values which I use even today .... and by the way my 1972 network is much more efficient and simpler than that used by , say, Polytone, Crate, or most others, even today. ;)
QuoteTriumph??? Jordan???
Well, at least those Triumph amps were the main amps in Keith Richards' rig
(not to mention the company built amps for Jennings/Vox) and I'm sure some other people were acquinted with them too...
(http://farm3.staticflickr.com/2624/3978974559_5291a2b656.jpg)
(http://farm1.static.flickr.com/38/109262279_29baac7173.jpg)
I'd imagine Marshall was more than willing to do some some research with whom and what they compete with.
Quote from: J M Fahey
So I stick to my guns about having independently invented and applied mixed feedback technology in 1972.
No argument from me JM, quite a few things have been invented independently in more than one place, Calculus for example. :cheesy:
Just to note that the RCA scheme I posted above is a bridge that actually allows the current feedback to be varied through zero to either phase.
Weell, I see that Jordan has been pulled out of the picture. Interesting.
Let's continue with the remaining contestant: Triumph
1) I see Jimi courteously explaining some licks to a friend or whatever, on a guitar that looks like an italian made Vox (maybe now we should consider it his main axe too?) ... which might be his or the other guy's, for all I know.
Any pictures of Jimi *actually* playing Live or in Studio with a Triumph?
2) SS Triumph amps the *main* amps in Keith Richard's rig?
Not so sure.
I'm talking about Keith Richards the Guitar player, of course.
Now if we talk about Keith Richards the BASS player, well, anything is possible.
Anyway, I'd *love* to see (40 years later ) the Triumph schematic.
Thanks in advance.
I have a question you guys probably would be able to answer. I have a 1978 Peavey Pacer 1-12 combo that I am trying to get into playing shape (just for kicks). On it's own, it doesn't sound too bad but, I plan on using pedals. (Wampler Plextortion & Wampler Tweed '57) for dirt. It has built in reverb that isn't so good & I thought about having an Accutronics long spring type of reverb put in it. I had the pots cleaned recently & thought I would also upgrade the speaker too. The back panel says 45 watts RMS- 8 ohm & I ordered a Celestion Patriot Black Powder 8 ohm- 75 watt. I haven't installed it yet but, wonder if this speaker is adequate to use. Also, is it safe to use pedals slightly louder than unity gain ? Thanks for sharing what you may know. Eric
Quote from: J M Fahey on May 27, 2012, 12:40:54 AM
Weell, I see that Jordan has been pulled out of the picture. Interesting.
Let's continue with the remaining contestant: Triumph
1) I see Jimi courteously explaining some licks to a friend or whatever, on a guitar that looks like an italian made Vox (maybe now we should consider it his main axe too?) ... which might be his or the other guy's, for all I know.
Any pictures of Jimi *actually* playing Live or in Studio with a Triumph?
2) SS Triumph amps the *main* amps in Keith Richard's rig?
Not so sure.
I'm talking about Keith Richards the Guitar player, of course.
Now if we talk about Keith Richards the BASS player, well, anything is possible.
Anyway, I'd *love* to see (40 years later ) the Triumph schematic.
Thanks in advance.
Hum Triumph, Was it not connected with Selmer or Vox in some way?
I remember reading that some SS Vox Amps where made in the triumph factory.
Anyway I just found this; http://vintageamps.com/plexiboard/viewtopic.php?f=10&t=78014
Pics are gone and I don't know if it refers to SS.
Ed; maybe it is SS read this first; http://vintageamps.com/plexiboard/viewtopic.php?f=4&p=373589
Phil.
Quote from: Eric D. Larson on May 27, 2012, 02:22:17 PM
I have a question you guys probably would be able to answer. I have a 1978 Peavey Pacer 1-12 combo that I am trying to get into playing shape (just for kicks). On it's own, it doesn't sound too bad but, I plan on using pedals. (Wampler Plextortion & Wampler Tweed '57) for dirt. It has built in reverb that isn't so good & I thought about having an Accutronics long spring type of reverb put in it. I had the pots cleaned recently & thought I would also upgrade the speaker too. The back panel says 45 watts RMS- 8 ohm & I ordered a Celestion Patriot Black Powder 8 ohm- 75 watt. I haven't installed it yet but, wonder if this speaker is adequate to use. Also, is it safe to use pedals slightly louder than unity gain ? Thanks for sharing what you may know. Eric
Hi Eric,
A big fat huggy welcome.
The speaker will be fine though it may alter the sound/tone a bit.
Reverb,, don't waste money on a fancy tank as the likely culprit will be the electronics that drive it all. You seem to like pedals so just go find a good digi rev pedal and save the pain of completly redesigning the rev section.
Pedals,, Assuming the Amp is in good working order then highly unlikely that any damage will happen to any amplifier by winding all the pedal knobs to 10.
Go for it.
Phil.
Phil, Thank you for the nice welcome & advice. As you may or may not know, there is a control on the front of the Pevey Pacer amp that says overdrive. It is subtle but, it works. I'm wondering if there is a way to change the curcuit to make this control become more effective & intense ? I also have an old Peavey Decade which to me sounds amazing - (much smaller amp but far more dirt capability) it has 2 inputs, one clean & one called saturation. Is there any way to use the pre and post gain controls (curcuit) from the smaller amp & put them into the larger Pacer to essentially make a larger version of the Decade married to the power section of the Pacer ? This would seemingly eliminate the need for a dirt pedal altogether. Just wondering. Thanks, Eric
Eric, I've played through a Pacer before (my buddy across the hall in the college dorms had one) and to my ear, that brand of mild, kind of greasy-skanky "Lynyrd Skynyrd-esque" distortion is great for all kinds of rock and blues, but to appreciate it you sometimes have to hear it in context. In other words, it might sound pretty lame to you if you just got done rocking out through a Boss Metal Zone pedal or using a Marshall JCM900 in a room by yourself...but when you hear that Peavey grit in a mix, it can really cut through, carve out a spot for itself to be heard without stepping all over the bass, vocals or other guitars.
Having said that, I've had great results with my slightly newer Peavey Audition 20 (early 1980s), which probably falls somewhere between your Pacer and Decade in terms of available dirt level, by setting it pretty loud and with a medium gain setting, so playing soft sounds pretty clean and playing hard gets noticeably dirtier. Push the output volume and speaker hard. And if you have it set like this and in front of it you place a clean boost pedal (I use an old beat-up DOD FX-10 Bi-Fet Preamp, but there are many others...some distortion pedals can also function as a clean-ish boost) you can get some great tones by having the amp on the verge of crunch, then pushing it into freakout zone by hitting it with a hotter signal.
I also like using a compressor pedal in front of the amp, set at a pretty mild, conservative level. I don't want to hear a special-effects squashed sort of compressed signal; I just want to hear a slight limiting on the peaks, and a slow enough attack that my pick attack comes through a bit...this seems to approximate the slight sag and compression that a tube circuit might display at the onset of overdrive.
Anyway, try those things before chopping into either of your amps...you might find a combination you like.
QuoteI remember reading that some SS Vox Amps where made in the triumph factory.
Yep, they were a subcontractor to the company that owned the Vox trademarks. They designed a few models and manufactured some other. If I remeber right they designed the 7 and 4 -series hybrid amps and the PC board AC30's.
QuoteAnyway I just found this; http://vintageamps.com/plexiboard/viewtopic.php?f=10&t=78014
Pics are gone and I don't know if it refers to SS.
I was going to post that when JM Fahey asked for the schematics. I have those schematics saved somewhere but I have an ungodly pile of CR-ROMs full of schematics in no sorted order and it takes a good hour to start browsing through the "suspects".
And for what...? A schematic of a generic solid-state power amp with generic current feedback scheme. All you knowing what the talk is about know what to expect. There's really nothing to see; the only remarkable feat is how early they did it.
Quote from: J M Fahey on May 26, 2012, 10:53:07 AM
With those parameters in mind , it was easy to calculate values which I use even today .... and by the way my 1972 network is much more efficient and simpler than that used by , say, Polytone, Crate, or most others, even today. ;)
Juan, I still don't count you on this until you start selling these in the USA. Bring on the quality Solidstate, discrete electronics based amps!
Thanks for the advice mexicanyella ! Gonna do that !
Also, speaking of Peavey Pacers, check out what this guy on the steel guitar forum did; built a finger-jointed pine cabinet for a Pacer chassis and two 8" neodymium-magnet speakers.
COOL !!!!
I LOVE when these trusty workhorses are kept living (and improveed), instead of sending them to the dump.
Quote from: J M Fahey on November 06, 2011, 08:08:19 AM
I noticed that effect on my own a ***long*** time ago.
I started making guitar amps in 1969 , all tube of course.
In 1972 Argentina defaulted on its external Debt (what Greece is about to do now) and, not having U$$ available, imports were impossible.
Tubes dissappeared from the shops, just like that .... or were worth their weight in gold.
Started using Transistors, which were still affordable, but noticed that the sound was not the same.
Part of it was that SS amps had too high damping (approx. 100 ) which caused "dry" bass; I measured my Twin Reverb type tube amps, and damping was around 1.
I added current feedback (straight from SS design books) until I got the same value.
It helped a lot.
I guess I invented "Valvestate" on my own, about 15 years earlier than Marshall, go figure.
Necessity is the Mother of Invention.
Hello Gents,
Just joined to make a few comments. You might guess who I am from my username. :-)
It was almost impossible for Jim Marshall to have known about CF, as he didn't even understand valves. Until he started to employ decent designers post 1988... nothing very revolutionary came from this stable. So it was his new design team that introduced CF to the Marshall brand!
CF circuits were used as the basis for 'reverb' drive circuits designed by Accutronics back in the early days of tranny amps.
Quote from: teemuk on May 26, 2012, 09:26:44 AM
I think it really falls down to how each individual perceives certain characteristics in tone.
Personally, I hear the effect only as slight boost at low and high frequencies. I usually have to struggle to hear it as well.
Then again, I've encountered people who say it makes an astounding difference. Last one saying so wasn't even talking about current feedback per se but about differences of running a 100% tube amp to either purely resistive dummy load, or to a dummy load that mimicked speaker impedance. The effect / difference is essentially the same.
For him the resistive dummy load was too sterile, the reactive was "squishy" and responding to playing dynamics making a "Night/Day" difference. Personally, I had to struggle to hear the slight difference. ...as usual.
I once angried someone by stating that only thing the "Reactance" control in his Rocktron Velocity did was basically equivalent to diming bass and treble controls of a generic HiFi -style EQ - nothing else. He got mad insisting the control turned his amp to touch-responsive dynamic setup that sagged like a real tube amp. ...all that from a generic boost of low and high frequencies. The control didn't even try to mimic the unique response of a poorly damped amp driving a loudspeaker. It just introduced a basic treble and bass boost.
It's all in how you perceive things, and perceiving can be based 99% on imagination and 1% on "real" auditory information. It's always more or less subjective. Therefore I wouldn't jump to definite conclusions too quickly. Yes, objectively viewed the damping barely has a slight effect on frequency response... but so far I never encountered anyone who would sense things 100% "objectively".
CF is more about the control of the speaker than more treble or bass... although that is also perceived. It's allowing the speaker to add 'mechanically generated' harmonics. At frequencies above 1000hz approx, the HF response of a typical guitar speaker is largely synthesised... not natural HF sound coming from the amp. This is produced by 'cone break-up modes,' which are characterised by the somewhat jagged speaker response curves above 1000Hz seen in speaker manufacturers publications.
Most players think of a speaker as reproducing the sound made by any amplifier. Which is incorrect to a high degree for tube amplifiers and modern SS amps with CF. In fact, the amplifier does produce a controlling signal, but the guitar speaker simply uses it as a motive force and, basically, interprets that signal as a 'new sound' containing those harmonics which did not come from the amplifier at all!
The art of good guitar tone is understanding these 'beneficial defects' in the components you use. When you understand what is going on in the complete article, then it is possible to exagerate them or subdue them, to whatever your sonic aims are. This is exactly why there are so many speaker models available on the market.
A great guitar sound wil be determined mainly by the speaker and how it interacts with the OPTX and cabinet loading. So CF is only the 'enabler', whereas the speaker really creates the sounds you actually hear... not so much the trannies or valves, or any other component to any significant degree for that matter. Without CF in a tranny amp, the speaker is effectively just a resistor that makes a noise in line with what it's fed; who's loudness is proportional to its changing impedance with frequency, unlike a compensating transformer output stage.
The designer plays a big part in this to. If a designer is into metal... then he will not be able to design a good country, blues or any other type of amplifier. As with any 'niche' market product, the designer wares the true assets in the form of his knowledge on how to bring all the convoluted defects together and achieve a desired outcome.
It ain't just about the technology or circuitry used! The understanding of how the sound is made is more important than being a superb electronics technician! It's a black art... and so is designing guitar speakers!
Thanks for this explanation. Not so sure I understood all of this, but it's an interesting point of view.
I'm just wondering if these non linearities that are enabled either by a tube/output transformer or by a SS amp with CF are in certain cases what causes the dynamics of the playing to be exagerated.
Wouldn't say for sure I have a comprehensive understanding of what "linear response" to dynamic playing should hear like, but I'm usually pretty good at producing the sound (both tone and volume) I want with accoustic guitars and at controling the distortion level of a crunching preamp with just how hard I attack the strings. Fact is, on some tube combos pushed in there usually prefered volume range, I often struggle to control these dynamics, generating barely audible sound when plaing soft and ice-pick-through-the-ears far too loud notes when picking just a bit harder. Just in the interest of better understanding, could you tell me if the dampening factor might play a part in this?
Quote from: QReuCk on July 24, 2012, 05:21:46 AM
Thanks for this explanation. Not so sure I understood all of this, but it's an interesting point of view.
I'm just wondering if these non linearities that are enabled either by a tube/output transformer or by a SS amp with CF are in certain cases what causes the dynamics of the playing to be exagerated.
Wouldn't say for sure I have a comprehensive understanding of what "linear response" to dynamic playing should hear like, but I'm usually pretty good at producing the sound (both tone and volume) I want with accoustic guitars and at controling the distortion level of a crunching preamp with just how hard I attack the strings. Fact is, on some tube combos pushed in there usually prefered volume range, I often struggle to control these dynamics, generating barely audible sound when plaing soft and ice-pick-through-the-ears far too loud notes when picking just a bit harder. Just in the interest of better understanding, could you tell me if the dampening factor might play a part in this?
CF will not present you with a solution to your endeavours sadly. It is so easy to get wrapped up in technical 'stuff' that might bring hopes of an instant 'playing' problem cure. Guitar sounds, in any genre, are helped mostly by the ability of the player and many years of experience of playing. Particularly in a live situation where 'feeling' the sound is so important and balancing pick pressure against drive and volume is a real skill to be learned.
Trying to abtain the characteristics you seek in a domestic environment might not bring much fruit. Attempting to recreate recorded sounds are too, a waste of time. They are mic'd with microphone(s) (there may be two or three used in various placements to create a sound) that add their own harmonics to the tone, then EQ'd by an EQ that works at different frequencies to guitar amp EQs, compressed... need I go on? A complete non starter! Not even the original artist can copy his recorded sounds exactly!
I know some very clever electronics designers who cannot design a decent guitar amp. Designers should have 'experienced guitarist' on their CVs to qualify. Many of the 'nonlinearity' and other attributes that pure technicians bestow upon guitar amps, although measurable, provide very little audible difference to a guitar amp tone. By the same token, I know many excellent players who just cannot find a great sound, even with boutique equipment. They either have it or they don't, or they need to practise more playing live. I guess... to be brutally honest. Most really experienced players can get a reasonable sound out of almost any old amp... on demand. CF is not the panacea. But it does help a tranny amp to sound much closer to an amp with an output transformer.
Ice-picky tone at volume is normal. The human hearing responds better to high and low frequencies at higher volumes (see Fletcher & Munson's work). You should EQ the amp for the volume you're playing at! Amps with a cheap output transformer, like guitar amps, also have narrowing bandwidth ability as they become heavily driven/magnetised. Hence Peavey's 'T-Dynamics' circuitry. But you can simply EQ the amps for loud playing... always refer to what your ears are telling you!
Hope this is helpful.
Fear you didn't understand me fully, or my limited english striked again and I didn't explained properlly. I am not trying to say I cannot obtain the tone and dynamic response I want from my gear, which happens to be a US made Peavey Envoy (or from any tranny I had a chance to test for that matter), whatever guitar might be plugged in (and this includes a nylon accoustic powered with a simple piezzo, a high output level Yamaha RGX and a more common Les Paul type electric).
What I am saying is that when testing some tube amps, I often have to drastically refrain my playing dynamics in order for the amp not to exaggerate it. I am also often very happy with how pure accoustic instruments respond to the variation of string attack I make.
Sure I didn't try the Fender Blues Junior long enough to fully adapt my playing, but I really chocked me how any attack just a bit stronger than usual resulted in a sound both considerably louder than what I expected and with a lot of unpleasant harmonics. I know how to EQ out these harmonics, but then softer parts would just be hidden behind the mix. I surely don't have these problems to that extent with a lot of other gears I tried and that includes trannies, but also oversized tube combos.
I am just curious what could explain such a difference and thought this dampening thing might have something to do with it.
Edit:
I just had the opportunity to test the BJ longer. Interestingly enough, some tweaking of the Volume and Master knobs enable to lessen the booming effect and dynamics were far more controlable. I guess a lot of components/features are interacting to influence the end-result. Still on my quest to understand what happens.
Quote from: mojah on November 04, 2011, 09:00:51 PM
I looking at adding a dampening factor adjustment to one of my SS amps. I'm looking at loosening up the feel a bit and before I put it under the soldering iron I thought I would ask around here.. My first impression would be to add more series resistance in the neg feedback loop at R108 any other thoughts?
Looking at the PDF, I see your Bandit is loaded with piece of crap RC4558 op-amps (if we are to believe that the schematic gives the part number that is actually installed). Nobody in their right mind would use this for a serious piece of audio gear.
The explanation for this is schematic date: 1989!
The first thing I'd do is replace all these with something for audio quality, like LM4562 or NE5532.
In 1989, an NE5532 was maybe still a bit of an expensive part for an amp like this, and the LM4562 was nearly 20 years away from development.
After putting in better chips, I'd play the amp and re-evaluate if it's worth it to muck around in its current feedback structure.
Quote from: Roly on May 26, 2012, 11:28:36 PM
Quote from: J M Fahey
So I stick to my guns about having independently invented and applied mixed feedback technology in 1972.
No argument from me JM, quite a few things have been invented independently in more than one place, Calculus for example. :cheesy:
Just to note that the RCA scheme I posted above is a bridge that actually allows the current feedback to be varied through zero to either phase.
Variable Damping Factor Control by Charles A. Wilkins, Audio, September 1954
http://www.aikenamps.com/vardamp.pdf
Quote:
"A voltage is developed across the 0.47-ohm and the 0.27-ohm resitors that is proportional to the current flowign through the speaker load. This current-proportional voltage is sampled by the slider on the 25-ohm potentiometer and fed back through low-pass filter RC as current feedback to the cathode of the first stage where it is added with the over-all negative voltage feedback.""Added with the overall negative voltage feedback" means "mixed feedback"!
Quote from: QReuCk on July 24, 2012, 08:33:19 AM
Fear you didn't understand me fully, or my limited english striked again and I didn't explained properlly. I am not trying to say I cannot obtain the tone and dynamic response I want from my gear, which happens to be a US made Peavey Envoy (or from any tranny I had a chance to test for that matter), whatever guitar might be plugged in (and this includes a nylon accoustic powered with a simple piezzo, a high output level Yamaha RGX and a more common Les Paul type electric).
What I am saying is that when testing some tube amps, I often have to drastically refrain my playing dynamics in order for the amp not to exaggerate it. I am also often very happy with how pure accoustic instruments respond to the variation of string attack I make.
Sure I didn't try the Fender Blues Junior long enough to fully adapt my playing, but I really chocked me how any attack just a bit stronger than usual resulted in a sound both considerably louder than what I expected and with a lot of unpleasant harmonics. I know how to EQ out these harmonics, but then softer parts would just be hidden behind the mix. I surely don't have these problems to that extent with a lot of other gears I tried and that includes trannies, but also oversized tube combos.
I am just curious what could explain such a difference and thought this dampening thing might have something to do with it.
Edit:
I just had the opportunity to test the BJ longer. Interestingly enough, some tweaking of the Volume and Master knobs enable to lessen the booming effect and dynamics were far more controlable. I guess a lot of components/features are interacting to influence the end-result. Still on my quest to understand what happens.
Hi QReuCk,
Quite frankly I hate to death some of the peavey bandit or similar Amps and some are EXTREMELY harsh.
In Fairness there are many many models and I don't have a warehouse full of every amp ever made so some maybe better than others. Often it's what you get used to hearing, what style you play and how you play.
For the guitar player who wants to play chords and have lots of crunch will want a different response than a shredding player who rates his ability by notes per second. :duh
FWIW, here is my observation
The conundrum you refer to can be easily explained by the simple fact that 90% of the greatest Amps ever made have quite extreme *Band width limited* output. (Not HiFi Flat)
Some of this is due to circuitry quirks and some guitar pu's that where used and of course the speaker has a great impact.
A lot of those Small valve amps tend to be mid/high heavy and distort like crazy even at low volume/gain,, prolly not much use for Acoustic.
In my experience I've had far more luck by limiting the bandwidth and watching how each stage effects the EQ.
In a studio mixer situation a flat response input may well be a better starting point but for guitar Amps a dead flat hifi response is almost always going to sound crap.
If you are using an Acoustic with on-board preamp and piezo PU then these often have extreme hi freq response which often come out sounding harsh with far too much harmonic content (Too much B/width)
As an experiment insert a distortion pedal after the Acoustic and crank it high into peavey or similar and then without touching any dials,,, swap it for an Electric.
I doubt it will need any explanation as generally the electric (Passive magnetic pu) will,,, by design roll off the very high frequency and sound a little more pleasing to the ear. winky.
Piezo pu's have extreme hi end response while magnetic pu's are almost opposite.
Bandwidth and Tone shaping are super critical for the style of music you wish to play.
Sadly there is no easy one line answer to this situation as there are many ways that will end in the same result.
As a rule of thumb;
If you want great electric guitar sound don't use equipment that claims extreme 20/20 bandwidth response.
So it's rather obvious that if you want a good R&Roll Amp use a crap design.
On the other hand if you want to reproduce every string scratch from an Acoustic you may prefer the more hifi approach.
Either way wide bandwidth is always going to be harder to refine and dial in whereas a limited B/width will be easier to handle for most situations.
Phil.