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Author Topic: Preamp needed... which one to build?  (Read 5993 times)

username

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Preamp needed... which one to build?
« on: November 04, 2013, 08:51:34 PM »
Hi guys,
I'm planning to build a transparent, clean sounding stereo preamp...
...why?:
I'm running my pedals directly into a Marshall 8008 Poweramp. I really love the clean sound of my guitar when the poweramp isn't turned on more than 25%.. the guitar- signal is just buffered by my pedals (no preamp)...
because I use no preamp the signal doesn't really get louder as i turn up the volume on my poweramp... it just starts to sound awful.
(It's a shame that i'm not able to annoy my neighbours with a half stack... :grr)

What I'm planning to do:
I'm planning to build a stereo preamp which should just add volume to my guitar signal without influencing the sound of the poweramp too much.

I've been looking for such a preamp- design on the net, but couldn't find any which looks suitable. Does anyone of you guys know what could fit my needs?

thanks for reading,
max

phatt

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Re: Preamp needed... which one to build?
« Reply #1 on: November 05, 2013, 05:36:37 AM »
Hi username and Welcome :tu:

If the signal is clean with the volume down low on the poweramp and you do not alter the pedals when you dial up the power amp then the signal is likely already to big and a preamp may not help much.
Maybe try and borrow a known clean preamp to test before you spend money.
Even an mp3 player signal might give you a clue as to how hot the preamp signal should be to get a loud clean sound.
As it is a dedicated guitar power block there maybe some kind of poweramp limiting inside the 8008 to actually add distortion as it cranks up and you may not be able to disengage that.
Others here will likely know more.
Phil.

J M Fahey

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Re: Preamp needed... which one to build?
« Reply #2 on: November 05, 2013, 06:12:05 AM »
What Phatt said,
Quote
because I use no preamp the signal doesn't really get louder as i turn up the volume on my poweramp... it just starts to sound awful.

what you describe sounds like clipping ... power amp clipping.

Just to make sure, set power amp volume very low (say 1 or 2), raise pedals own volume until *they* start to clip and then back down a little, so it's clean again.
Now you set your pedals for the loudest clean signal.

Then start raising *just* the power amp volume.

When it starts clipping/distorting, it's reached its maximum headroom or clean level, pot number by itself does not mean that much.

If it starts clipping on 2 or on 8 , it does not matter, the fact is that it has reached its limit.

Can you explain your setup better?

I know the 8008 well, but "pedals" is quite vague.

Also tell us about your speakers.

The 8008 had a "Flat/Guitar" switch, test it both ways.

*Some* had a bridging option: do you use it?

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Re: Preamp needed... which one to build?
« Reply #3 on: November 05, 2013, 12:51:51 PM »
Hi guys,
thanks for your fast replies...

...so...
my guitar goes through the pedals directly into the Poweramp. When I play clean, all the pedals are OFF, so they work just as a buffer (The guitar directly into the poweramp sounds too awful)

I used to own a Digitech Hafler T3 Rack Preamp. With it my Poweramp and Cab was driven to "loud as hell", but I didn't like it's sound so I sold it...

What I'm looking for is a simple (preferably Solid State) design to build an clean Preamp to drive my Poweramp...


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Re: Preamp needed... which one to build?
« Reply #4 on: November 05, 2013, 12:52:48 PM »
Would this buffer section http://www.muzique.com/images/buff4.gif
into this preamp section http://www.till.com/articles/GuitarPreamp/images/preamp.gif
provide enough volume increse?

J M Fahey

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Re: Preamp needed... which one to build?
« Reply #5 on: November 05, 2013, 10:46:24 PM »
No.
The buffer is not needed and the Tillman preamp has very low gain (around 2X) and little headroom ... like anything powered by a single 9V battery.

Go for Rod Elliott's P27 project preamp.

Roly

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Re: Preamp needed... which one to build?
« Reply #6 on: November 06, 2013, 05:48:43 AM »
Hi @username and Welcome.

Quote from: username
The guitar directly into the poweramp sounds too awful

I'll bet it does.

Looking at the circuit of the 8008 a couple of things stand out; firstly that the input impedance is only 5k ohms for Input B (or 10k ohms for input A) which very few guitars and only a few pedals would be happy driving.  I would expect it to sound both thin/trebly as well as  weak.

Typical guitar amp inputs are more like 1 megohm (1000k) and pedals generally like 50k or more.

The second thing is that these amps don't appear to be particularly sensitive and look like they need more than a volt input for full output, and the only way you'll get a volt out of a guitar is by belting it or dropping it (and much less so into only 5-10k).  Fully driven even one channel of this 80 watt power amp should be ear-bleeding loud or "loud as hell" as you've found.

You still haven't specified your pedal train or speaker setup (how many, what size, open back or sealed?).

The muzique buffer will certainly increase the impedance but it won't give you any voltage gain at all, and the till buffer will also give a small amount of gain but not as much as you seem to need, about x2-x3 when you need more like x10 (or more if you want something in reserve).

Like JM I suggest Rod's P27 preamp.  This has a 1Meg input impedance and more than enough gain and output level to fully drive the Marshall amp, and includes a Fender-ish tonestack.

If you order a board from Rod you also get access to more detailed construction information and personal backup.  (Disclaimer: I have no connection with Rod but know a good design when I see one.)

As you want stereo get two (just about any +/-15V supply will run both) and you can either use dual pots or have independent controls for each channel.  I run a homebrew Twin-50 with independent controls, and while this is a minor pest there have been many occasions over the years where having two totally independent amps has been very handy.

HTH

If you say theory and practice don't agree you haven't applied enough theory.

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Re: Preamp needed... which one to build?
« Reply #7 on: November 18, 2013, 09:01:53 AM »
Hi guys,
thanks for all the helpful information... I think I'll go with the P27 preamp...
I've just replaced the burnt R8 resistor and the C12 cap which act as feedback oscillation or so...
Now the poweramp barely clips, even when turned up all the way...

My Setup: Guitar--> Big Muff --> Boss DD7 (Stereo Out) -->Marshall 8008 A Channel--> 4x12" Cab driven Stereo,
                                                                            -->Marshall 8008 B Channel --> which provides 4 Ohm per side

cheers,
max

Roly

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Re: Preamp needed... which one to build?
« Reply #8 on: November 18, 2013, 12:45:23 PM »
Quote from: username
I've just replaced the burnt R8 resistor and the C12 cap which act as feedback oscillation or so...

That's a bad sign; that's the safety ground lift resistor (to prevent ground loops) and its bypass cap.  It suggests that the amp got a mouthful of mains current between its signal ground and mains ground (or got hit by lightning).

In any case, because it concerns a possible earth fault, most likely in some other gear that was connected to it at some stage, it's pretty important we try and track back when, what, and how this happened.
If you say theory and practice don't agree you haven't applied enough theory.

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Re: Preamp needed... which one to build?
« Reply #9 on: November 22, 2013, 06:28:29 AM »
Quote from: username
I've just replaced the burnt R8 resistor and the C12 cap which act as feedback oscillation or so...

That's a bad sign; that's the safety ground lift resistor (to prevent ground loops) and its bypass cap.  It suggests that the amp got a mouthful of mains current between its signal ground and mains ground (or got hit by lightning).

In any case, because it concerns a possible earth fault, most likely in some other gear that was connected to it at some stage, it's pretty important we try and track back when, what, and how this happened.
Hi,
I guess they already were burnt as I bought the poweramp (bought it used)...
...but as I said I replaced them... shouldn't be a problem yet? or do you think that anything else might be defect too?
thanks for all the info, again
cheers,
max

Roly

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Re: Preamp needed... which one to build?
« Reply #10 on: November 22, 2013, 12:41:18 PM »
Okay, then I think that was just what it was, a residual from something back in the history of the amp.  You've fixed it and there is no other reason to think the amp is damaged(?), so you can concentrate on your preamp.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.