Hello,
I'm quite new to amplifier tweaking/building. I recently serviced a polytone i bought used for 25 euros and that got me thinking about building a polyclone.
The amp i have does follow exactly the schematics on the murchmusic site.
However, i read that this power amp was difficult to bias and was not really easy to build. Another guy that build a polyclone told that he replaced it by a FET power amp.
How much does a power amp change would change the sound of the amp?
Are there classical and good sounding power amp designs around 80 or 100 Watts that would preserve the polytone sound?
In general, are power amps transparent and does the color of the amp comes from the preamp?
Thanks in advance for your help
Just my opinion, others may disagree:
In general, I don't see the power amp of a solid state instrument amp being a real part of the tone shaping. I expect that to come from the preamp. In the case of a combo amp, the single thing that has the most effect on the sound is the speaker. And that includes its enclosure.
If I were cloning a Polytone, I'd probably just dummy up the preamp, and either just buy a commercial power amp or if I built something, I;d probably go with a chip amp like an LM3886 based project.
Or I might take an old workhorse like a Peavey head with a 400BH power amp and build a preamp on the front of it. Or take the 400BH board from it and use it in my clone.
that's what i read earlier and the confirmation i wanted.
So there is no hearable difference between power amps or different class and using different technologies (FET vs BJT vs chips vs ... )?
so basically, the great sound of a polytone comes from a super simple opamp preamp with a bandaxall eq, and a crappy speaker with no brand and name written onto it? could it just be the bandaxall eq with flat mids that make a big part of thatgood sound?
Yes, I think as an overly simple generalization that is the case. I will say there are some subtle differences in amps, but not really the tone thing. I like the effortless sound of the old Trace Elliot heads, which I attribute to the MOSFET output stage and its low resistance. But overall, I can;t hear the difference between the Hartke 7000 MOSFET or the Hartke 7000 bipolar amps. ANy difference between those power amps in the otherwise identical amps is tiny compareed to the preamp.
This is a bit of a can of worms and harks back to the transistors vs tubes wars in days of yore, but I agree with Enzo and would add that the output stage in a typical solid state output stage has so much excess gain and so much negative feedback applied that below overload/clipping it makes very little difference what circuit or devices are used; they are all effectively transparent.
Once they clip it's a whole other story, but it is normal for solid state amps to introduce clipping in a controlled way in the preamp (or as a stomp) and to operate the output stage below clipping - then the tonality (if any) is determined by the preamp.
Looking at the circuits (attached) this looks like a pretty conventional amplifier.
Their power amp is quite conventional apart from the application of current feedback (R20 to R23), and while I'm not mad on the method of bias adjustment it should still be straightforward.
The only really notable things about the preamp is the use of thumpless LDR channel switching, and a fully asymmetric clipper in the drive channel. The input impedance is 150+680= 830K for Hi and around 600K for Lo.
Of all the stages the Drive clipping stage, here U4, plays the major role in the tonality of a preamp; the number and type of diodes used, and how they are applied to the guitar signal.
Two other important factors in the overall sound are the speaker cabinet, and preamp input impedance.
It is fairly easy to cross-connect different amps and speakers, and you quickly discover that, generally speaking, different cabs make much more difference to the sound than different amps.
The preamp input impedance is important when a passive guitar pickup is used straight in. Again as a generality, most amps (and stomps) have an input impedance that is too low and damps the natural resonances of a passive guitar, thus tending to make all guitars sound similar.
The tonal character of the reverb is determined to a large extent by the reverb springline tank itself (normally a third-party item), and to some extent by the drive circuit used (voltage or current drive).
HTH
wow thanks for the great answer.
So in this case (my polytone doesn't have the drive section, nor the reverb section, only the basic amp/EQ preamp, and the exact same poweramp that is on the schematic). it is safe to assume that most of the tonal characteristics of the amp come from the bandaxall eq with flat mids, the bright switch, the speaker and cab ?
I'm curious about the heatsinking of this amp.
The 2 power transtore are mounted on 2 heatsinks, it seems ok to me, but the whole amp if filled with insulating foam, very light. i confess i worried about the heat resistance of this foam and wondered if it wouldn't start to burn if the power transistors started running too hot. is that standard practice?
Also, how do you guys make chassis? the biggest problem for me in this hobby is the hardware part :-)
The problem with most transistor power amps is that the Safe Operating Area protection circuits get activated when the amp clips with a speaker load. Driving a resistor dummy load is not a problem. The Polytone has no protection circuit other than a simple fuse. Drive it hard and it will eventually toast the output transistors. Most protection circuits are too simple to allow a big enough operating area unless the instantaneous voltage across the output transistor is limited to about 50V. There are some exceptions, and MOSFETs don't have this problem. Chips like the LM3886 have a much more complex protection circuit and they perform better. There is a graph on the LM3886 data sheet that shows the SOA. It's pretty impressive. Current limiting sounds bad. Voltage limiting is good. You need to stimulate a speaker with voltage and let the current go where ever it wants.
One other thing to look out for is overdriving the input of any chip. On the chip data sheet there will be an Input Common Mode range. Don't drive the inputs beyond it or the output will not obey the input. The output usually slams the opposite rail. It sounds really bad but you won't be able to diagnose it without an oscilloscope.
Custom chassis making is the most complex task, because tools are "industrial" and not available for the home maker.
Some suggestions:
1) forget steel, work with aluminum sheet.
1.25mm thick is a good homemaker compromise.
1mm is way too flimsy and 2 mm, although desirable, usually can't be bent at home.
2) it *can* be bent to right angles, using 2 pieces of 2" x 2" angle iron as clamps, the whole pressed in a shop vise, and applying judicious pressure (or some hammer blows) through a piece of wood and a hammer.
3) I worked for ages using just 2 drill sizes: 10 mm for old style jacks (the ones you find in old Fenders and such), "large" (25mm) pots, even found a switch which fit 10mm holes, power cables (with a rubber grommet) and 4 mm for everything else: 1/8" screws which held PCBs, transformers, power transistors, you name it.
Primitive but functional.
If, say, I fly anywhere with my hands empty, I can start building "sellable" amps in a couple days, with stuff bought at a hardware store.
This is an example of a 1972 200W RMS amp made with such poor tools:
(http://farm2.staticflickr.com/1161/729090782_cf3fb0f620_z.jpg)
All holes either 10mm or 4 mm, except power fuse holder and switch (13mm), which required a couple minutes of filing.
Quote from: add4 on January 09, 2013, 10:43:22 AM
I'm curious about the heatsinking of this amp.
The 2 power transtore are mounted on 2 heatsinks, it seems ok to me, but the whole amp if filled with insulating foam, very light. i confess i worried about the heat resistance of this foam and wondered if it wouldn't start to burn if the power transistors started running too hot. is that standard practice?
Also, how do you guys make chassis? the biggest problem for me in this hobby is the hardware part :-)
Lacking an internal pic I can only go from your comment, but some people seem to think that heatsinks somehow
magically dispose of heat. A heatsink is a
coupler between the case of the device and the surrounding air, and it requires a constant supply of cool air to heat up to dispose of the generated heat. If it is not mounted somewhere it gets a constant supply of fresh air it will simply recirculate the already warm air making it hotter.
Because heatsinks tend to be both expensive and a bit problematic to mount Murphy's Law of heatsinks is that they are often too small; you can't have too much heatsinking. Note the ones in JM's pic above, lots of fins and covering the entire back panel.
I'm at a bit of a loss to understand "the whole amp if filled with insulating foam" and can only imagine that this is some sort of misguided attempt to make the electronics more robust for transport, and I would expect thermal problems to follow.
Can you post a pic please so we can see what you are talking about?
A couple of alternatives to metal bending are to use flat sheets joined with L-section, and have a look at the range of ali extruded sections that are available, I've seen chassis built up of various sections bolted together.
The Polytone is a very compact combo amp, and closed back at that. The preamp is a small cut in panel at the rear of the top - controls face up. The power amp is on a small chassis in the bottom. Shielded cables connect the two. The speaker fills the front wall of the thing. The entire cabinet is stuffed with common pink fiberglass insulation. This basically a powered speaker. The insulation is stuffed in this amp for the same reason it is stuffed in any speaker. Nothing to do with anything thermal or with any sort of padding to the electronics. SImply damping material for the speaker.
Pain in the ass to work on? Yes, indeed.
Oh and chassis? They make them commercially too. I have never made a chassis for anything in my 50+ years of electronics. I always bought either ready made chassis, or I repurposed some existing one. COnsider that option.
What enzo described is exactly what i found into my polytone.
i posted gutshots here :http://www.ssguitar.com/index.php?topic=2771.0
No distortion or reverb in mine, just the very simple basic preamp. the preamp pcb has a lot of free traces and spaces and i guess i could add a reverb from this, but i don't see where i would place the pot and the reverb unit
I'd be curious to know where i could buy chassis, enzo, could you share some links?
Thanks to you all for your help !
You show the PCBs but not the fiberglass ;) :o
So I guess I'll have to show mine :lmao:
This guy clearly had an obsession with fiberglass.
Some people think that not only heatsinks magically draw heat away, but also that fiberglass magically turns a bad speaker into a good one. :loco :duh
This is a "big" Polytone (by their standards), 100W into 2 10" speakers.
*Very* heavy for its size, which sort of defeats its purpose.
In fact, I think they do not blow often simply because they are used by gentle Jazz players, but when driven loud, yes, they die.
Notice the ton of free space available on the PCB and missing parts ... and this is 2 channel, with distortion and reverb.
I guess they designed *one* big PCB covering all bases, and mount just what's needed.
EDIT: just for you to feel better: the Factory original chassis shown, also has "DIY" characteristics.
By itself it's the simplest one, a "C" folded sheet of thin aluminum.
Since it probably was too flimsy, they added on the ends the two silver shiny blocks ... made out of a square piece of plywood covered in kitchen type aluminum paper, glued with contact cement, how's that ?
PS: try to find some local tinshop, the kind who custom make gutters, air conditioning tubing, pig or chicken feeding trays, etc.
They have the required shears and folders, and maybe some punching machines.
Lightweight type, because they are meant for 18 Ga or thinner galvanized sheet, but that means they can easily process up to 2mm aluminum.
The chassis I showed above was made that way: "they" cut and folded it and I did the rest.
Never went empty handed.
I was "this weird nerdy kid with crazy ideas" but visiting them with a couple "longnecks" (good quality wine bottles) in my rucksack always made me welcome by the blue collar guys in the shop ;)
Or go the extruded aluminum route, visit some shop which makes custom windows, doors, office separators, etc.
Look at the catalogs, there are infinite cool shapes, pop riveting them (can't be soldered) will offer many possibilities ,,, and anodized, polished , scratched or blasted aluminum looks *good*.
Or go the Enzo route; get for peanuts or for free some dead amp and gut it.
Aluminum - and steel - chassis boxes are an industry commodity. Brands that come to mind are Hammond (the transformer people) and Bud Industries. But i am sure ther are many others. I spied a whole passel of them at Angela.com, a supplier to this industry, but general parts houses like Mouser, Allied, Newark, and most others will have them as well.
Chassis come in all shapes and sizes. Look at the power amp chassis in your Polytone, is it not just a rectangular box? And the preamp, the same, another rectangle? Chassis have an open side, but you can either cut sheet metal for your own cover, or they usually sell covers too. Chassis also come as project boxes. That would be a two part (typically) box that completely surrounds the innards - no open side. I am thinking basic sheetmetal chassis, but they also come diecast - like a lot of efect pedals are built in - rounded corners and a screw on bottom. And there are more decorative things like to put finished goods into
Here is a page from Mouser with a large selection of Hammond chassis. These are aluminum, but many come in steel as well.
http://www.mouser.com/catalog/catalogusd/645/2079.pdf
Here is the chassis page from Angela, note the plain boxes, as well as those ready punched out for guitar heads, and fancier stuff even.
http://angela.com/chassis.aspx
Here is the chassis page from AES
http://www.tubesandmore.com/products/enclosures
Those are just a few of many. I was buying aluminum chassis 50years ago, and they were not rare then either.
Great, thanks!
Another question : do you have a magical solution for producting small batches of PCB for a good price? do you etch at home or do you send them for production ?
Yep no worries,,,,,IF You've got a couple of million dollars to spare :lmao:
Might be cheaper and you will learn more if you take it all in steps.
you have to crawl before you can walk,, if you catch my drift? ;)
Phil.
I make PCBs at home, but I'm making them every week, and sell the amplifiers so I had to cut costs down to remain competitive.
That justified buying the necessary stuff and learning to use it.
So there are different options but "best" depends on how many you will make and for how long .
1) If you want to test your concept, maybe sell 1 or 2 to test the waters, go for a DIY oriented commercial fabricator, such as Express PCB:
http://www.expresspcb.com/ExpressPCBHtm/SpecsStandard.htm
Their service is incredibly useful for Robotics guys, Microprocessor users, etc. where a small but very complex PCB is needed.
Problem for us Music Instrument Amp makers is that we use large simpler PCBs and they charge by the square inch, so costs go through the roof quickly :(
2) Next best for small volume is using precoated photosensitive PCBs.
http://www.mouser.com/Tools-Supplies/PCBs-Circuit-Boards/_/N-8vofl?P=1yzvq68
Incredible Pro quality, sharp thin lines, you can leave the photosensitive coat on , which is solderable and protects the copper from oxidation, the works.
The raw board 300x450mm (about 12" x 18") which you can cut to size is more in line with what we need.
You'll need to design and draw your PCB artwork, print it on transparency or "vellum" (the semi transparent drawing paper used by Architects and Engineers) and photo expose that on the PCB sensitive surface.
Look at this tutorial:
http://sfprime.net/pcb-etching/index.htm
It's too corner cutting, he prints on regular paper and greases it with cooking oil instead of using the proper transparent papers, but it will give you an idea.
3) For small scale production at hobby level, thermal toner transfer is fine and inexpensive, but you must look very carefully at the printed PCB and correct with Sharpie , it's very easy to have small cracks or even parts of the tracks missing, because at some spot the toner didn't stick very well to the copper.
There's TONS of how-to guides, search for them, and practice practice practice :)
http://www.instructables.com/id/Cheap-and-Easy-Toner-Transfer-for-PCB-Making/
http://www.pcbfx.com/main_site/pages/products/transfer_paper.html
4) If you will be making them regularly, or need, say, 20 or more PCBs, learn silkscreening (whay I use) but you need to be really committed to it, spend some $$$ and learn to use it , practice makes perfect, so it really depends on how many you want to make.
If you have a friend who silkscreens TShirts and such, he already has most of the stuff plus the required skills, so if you buy the stuff he does not have (PCB printing "ink" is different , etc.) he might print them for you.
5) of course you'll have to download a PCB drawing program and learn to use it.
Eagle is popular, but the free version isn't that great for us, it's also geared for Microprocessor guys and such and allows for a too small PCB.
Download and try Express PCB even if they won't make your PCBs, it's relatively simple to learn and you can print your final drawing and use it for toner/photosensitive/silkscreen. Cool.
Only gripe I have with it, is that it's preset for too small pads and thin PCBs, we use much larger because of higher currents and mechanical vibration.
But it's a good learning tool.
6) last but not least: the ***OLD*** technique (I started using it ) of printing your design on regular paper, then you cut it, tape it over the copper side as a guide, and prick through each component hole with a sharp needle, as to mark its position on the copper.
Then you pull the paper and hand draw the lines with waterproof Sharpie or similar.
Crude but cheap and easy.
In this case you needn't even draw and print the proper schematic using a PCB, a 1:1 pencil drawing on any paper is enough.
Did I say **CHEAP**?
Hi add4,
What J M Fahey did not say is how many hundreds/ (make that thousands) of hours it took just to get where He is now. 8|
The best thing you can do is start with small pedal type circuits ,, simple and low voltage is safe for those wishing to indulge in electronics.
Better still purchase a solder less bread board setup and just plug in a few ideas and see what works.
For home hobby types,,By far the most expensive part is the box and the fancy front panels,, if that is the direction you wish to go.
For me I've got no labels on myself and I don't care much for fancy art work as the TONE is what I want not the name or the fame. 8) 8) 8)
I come in at number 6 of Fahey's ideas on how to make PCB stuff. :lmao:
For those starting out it works and does not cost a fortune to make at home.
As long as you don't go silly and try to make incredibly complex PCB's with insanely thin tracks it will work fine.
My first dozen PhAbbTone circuits where made by hand in the manner mentioned above.
I've not had one complaint from those that have purchased my home made handy work.
The biggest mistake you can make is to build yourself into a corner,, meaning building stuff you can't fault find because you do not have the technical ability to understand how it works.
(I am also guilty of that mistake) :-[ :-[ :-[ :-X :-X
As to chassis,, The trick is to find an industrial estate with Sheet metal workshops.
You raid their waste bins,, you can get lucky and find off cuts that are often close to what you need. (Wise to ask first)
I'm lucky because I spent most of my working life in the metal industry so I already know what to look for.
Failing that the net may well contain many ideas of how to work with sheet metal.
Steel is ok at 1.2 mm but over 1.6 it becomes a lot harder to bend at home.
2 mm is tough and is better left to the experts.
For the novice picking gauge of metal in a scrap bin will be tricky but it won't take long to note the subtle difference between 1.2 mm and 1.6 mm.
Aluminum is great stuff but a little harder to find as scrap and you only get one go at a 90 deg bend. :-X
Phil.
QuoteIn general, are power amps transparent and does the color of the amp comes from the preamp?
Depends. Many guitar amps are intently designed to NOT BE transparent. This including the Polytones.
As you can examine from the schematics the power amp includes a switch to toggle or disable the current feedback path. (As you can see, the speaker's negative terminal does not connect directly to ground, instead load current is converted to voltage by a low-ish resistance - two 0.3 -ohm resistors in parallel - and then fed back as another negative feedback input). This will consequently increase the amp's output impedance resulting into decrease of damping factor and variation in frequency response when the amp drives a reactive load such as a typical loudspeaker.
Disable this current feedback path and the amp will have a flat frequency response, like generic HiFi or PA amps that are - for understandable reasons - designed to not color the tone. Enable the (current) feedback path and the amp will begin to have more gain at higher load impedances, which will effectively provide a boost at speaker's resonant frequency and at higher frequencies where coil inductance increases the impedance. This is the kind of colouration introduced by many amps of low damping nature - most prominently tube amps.
So I'd say: with a good chance a modern solid-state guitar power amp IS NOT TRANSPARENT and will provide some forms of tone altering. Most of them use at least this particular scheme (and many today feature diode clippers for distortion or elaborate tube power amp emulation schemes e.g. Peavey's TransTube amps, Vox's Valvereactor amps etc.) The current feedback scheme in all it's simplicity - also found from many Polytone amps - has been featured in many transistor power amps since mid 1970's. Earliest examples actually even date back to earliest history of transistor guitar amps to begin with. IMO, these things have never been something that could be universally considered "transparent" or "non-colouring". Some may be, most aren't.
The power amp has a huge influence on sound, solid state or not.
A solid state voltage amplifier (global negative feedback based on output voltage) sounds nothing like a similar (or the same) amplifier in with mixed mode feedback (global negative feedback based on a mixture of the voltage applied to the speaker and the amount of current flowing through the speaker). The difference in response cannot be obtained with an equalizer. Creating the tone of mixed feedback in the preamp and then using a voltage amplifier falls into the territory of emulation, raising issues of whether it is true to life, whether it sounds right at full volume, etc.
Moreover, power amps can have tone controls which are in the feedback loop. If there is current feedback, then these controls interact with the speaker, and so they "do things" that a preamp EQ does not. For example, a bass control in the negative feedback loop will increase speaker resonance, which is not the same thing as equalizing for more bass.
Besides types of negative feedback we could probably talk about other possible effects that could happen in power amps, but that's one big one.