First amp advices?

[add4]

hello,

I am thinking of starting a first amp project and i'd like to have some advices from you experienced people.

First some background: i have been building guitar effects and 1 tube amp in the last 2 years. I have a general understanding of circuits and how some parts of them work, but nothing really complicated yet.
I usually play at designing my own pcbs in eagle when i see a schematic that i like. Usually it's much simpler than an amp, but nothing seems too complicated to be done with 2 layers and a few squares inches of pcb.

I have a solid state amp project that i'd like to concretize and i would like to have your opinion and advices.
The idea behind the project is to clone my beloved polytone mini brute.
The preamp would be a clone of the preamp of the mini brute (basically an op-amp amplification device with an active bandaxall EQ).
For the power amp, I've heard the polytone power amps are really hard to bias and basically a bad design. I've also read that power amps basically do NOT change the sound at all, they just amplify the sound. So i was thinking that i could move away from the polytone power amp for a classical design with more qualities. However, i would like to be SURE that i' wouldn't be loosing some of the key features of the polytone sound by doing that.. do power amp influence the sound/dynamics/feel of the amps at all?
Power supply : linked to the power amp i suppose -> ??
Dimensions : as small and light as possible .. my polytone packs everything in a very tight package, and 9 kgs.. i'm hoping to do that, or better as components are now smaller...


Here is what i already know that i don't know...
- The practical connectivity into the amp: how to send signals/power to the boards (if there are more then one), and switches.. what physical connections should be used and how.
- heatsink!!! i don't know anything about it. my polytone has 2 power transistors glued to 2 big heatsinks, which are stuffed into the CLOSED amp, filled with fiberglass foam .... so i guess heat dissipation is not at its best, still, it doesn't seem to matter.. i guess it's possible to do better, how to manage that properly.
- power supply needed : depends of the circuit and power
- where to order a decent and not too expensive faceplate
- the actual cabinet construction . not the slightest experience but i can be helped.

So my main question at this point is : would changing the power amp fro mthe polytone power amp to a classical design change the sound?
How do i calculate the needed heatsink and power supply ?

Thanks in advance

[phatt]

Your circuit already has current feedback in the power amp which is what many other brands have as well so unlikely you will benefit much from a new power stage. Waste of time really as all the tricks with SS rigs are done in the preamp stages.
Phil.

[teemuk]

I've also read that power amps basically do NOT change the sound at all, they just amplify the sound.

This entirely depends on how the power amps in question are designed.

I can tell you for instance that power amps in amps like Vox Valvetronix series or Peavey TransTube & Vypyr series do far more than amplify sound.

Most guitar amps do at least have current feedback, which at least makes the frequency response VERY non-flat. Advanced designs like Peavey's, Pritchard's, Quiolter's, etc. not only do that but may also soft clip, compress, hum and crossover distort like many generic class-AB tube amps.

Saying that power amps only amplify and nothing else is blatantly ignoring several decades of design dedicated to emulating tube amp behaviour with SS.

[J M Fahey]

Agree, but let me add that what the OP is asking is what can he replace his 35 y.o. technology amp with , while keeping that "old" sound.

Peavey/Vox/Pearce/Pritchard/Bluetone tube emulation amps are great but play in another league.

Polytone got a bad rep from one Power amp version based on an LM391 driver chip, a good idea on the bench which is not so good in practice .... adding to that that the chip is obsolete and hard to find.

But they also made discrete versions which sound exactly the same and are "buildable" .

So the OP can clone it or even modify a regular "hi fi" SS power amp and , if power is enough, even a chipamp such as LM3886 , as long as he uses the Polytone mixed feedback network which, as Teemu said, shapes the frequency response.

This is thye discrete one, which can be built easily, although I suggest replacing transistors with same sounding modern ones.

The online schematic is stupid upside down and was never corrected  :duh , I'm attaching a right orientation one, marking the important feedback parts, all else is conventional

[add4]

Actually my polytone uses that exact power amp so I guess that solves the problem.i thought that this power amp has the bias problems.

Actually, after trying many other solid state amps from vox to aer and henriksen jazz amps, I have never found an amp with the same reactivity (attack of the notes is super fast) and dynamics as the polytone.

Do you have any idea what could be the reason for this 'directness' ?

[add4]

At the moment I couldn't find any amp tube or solid state, with a sound as precise and immadiate as this polytone and I'm really really curious to know why as these are great features to me.

Te preamp is fairly classical so I guess it's a matter of power amp/speaker as the previous posters mentioned

[J M Fahey]

It's a combination of factors.
The preamp is a straight/flat audio/hi fi  type one, the tone control turnover frequencies are "wrong" for a guitar, correct for home audio use, so with a guitar the treble control is very weak and the bass one overpowering.
Which by sheer chance is what the Jazz cats want: couldn't care less about treble but love full sounds, not only bass but too much midbass too.
The proper speaker is a mid quality Eminence , no chinese cheapies *nor* Celestions (God forbid).Pick a 12"Legend and you won't regret it.
Combine that to a speaker in a very small hermetically closed cabinet and you'll have an important mid bass bump.
It should be boxy and nasal as H*ll, butbthen overdamping compensates for it.
So in a nutshell, the schematic is important, buy is only a part of the picture.
Only differende I'd do is to cut a "window"in the back panel, build a wooden "box" inside attached to it, and put heatsinks in that "cave" , so cabinet is still sealed but heatsinks live outside,semi embedded in the back panel.

[add4]

Well it's midrangey as hell indeed, but the real killer feature is the bright switch that brings a lot of highs back into the sound. all notes have bass in them too so they weight a lot in the mix. With the bright switch on the sound is actually very bright with mids Added. Think Kurt rosenwinkle sound on his YouTube video for lehle pedals.

But again if someone knows what gives that immadiate attack to notes I very curious of what makes it happen

[J M Fahey]

"It's a combination of factors ...... etc etc etc."
If you expected a simple answer such as " it's Capacitor C14 / cabinet wood type / etc." , well, reality is complex.

I might add that it's an SS amp, which usually are sharp/fast/tight.
But you already knew that.