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Old 16th October 2013, 04:20 PM   #21
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Quote:
Originally Posted by AndrewT View Post
40W of ClassA into 8ohms requires a peak ClassA output current of 3.2Apk

The quiescent current of the output stage would need to be set to ~1.6A to achieve that 40W, i.e. 320mA through each of those ten 0r56 emitter resistors. That is 179mVre

Could the bias be set that high?
it is already cleared up that this is NOT the DOXA 40.
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Old 18th October 2013, 12:01 PM   #22
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Hi all, and thanks for the input!
This is the amp I've got:
https://encrypted-tbn2.gstatic.com/i...-2y2a_7PzvvdFQ

The transformers have two secondaries each, rated at 31V/250VA. At present, the power supply is wired quite strange (at least to a solid-state novice like me): One leg of each secondary winding is hooked together at a common ground (which is not connected to the chassis at all, but only to the negative speaker terminal and the screen on the input RCA). These ground leads all gather at the serial connection point between the filter caps. These are 10.000uf/50V units, running two-and-two in parallel. The two banks of two caps are wired in series and it is at the connection point between the two that the gounds are gathered. The other legs of the secondary windings are connected to a bridge rectifier, with the cap bank connected between the DC output terminals. So the voltage accross the DC terminals on the bridge is 91VDC, but of course it's +/-45.5VDC if ground is 0. The circuit boards are fed the full 91VDC.
Is this the normal way to construct a PS for a solid state amp?

Again, what I want is an amp that runs a little cooler and a lot quieter. If it also has a bit more power, thet would be a bonus. In valve powered guitar amps, you would generally get a louder and cooler-running amp if you bias the output stage to run in class AB rather than class A. (Very few gutar amps are truly class A, but some cathode-biased designs run very close and very hot). I know that transistors are very different from valves, but thought that maybe the same principals apply.

Thanks again everybody! It's great that you're willing to help out newbies like myself.

Henrik
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Old 18th October 2013, 05:26 PM   #23
AndrewT is offline AndrewT  Scotland
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You and no-one else can get more power than what the supply rails will allow.
re-biasing does NOT give more power.

Your 91Vdc is your power limiter.
Expect an absolute maximum of ~30Vac into a high impedance load.
Into a normal impedance load your maximum is likely to be in the range 20Vac to 25Vac.
i.e. 50W to 78W into 8r0
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Old 18th October 2013, 06:19 PM   #24
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Quote:
Originally Posted by AndrewT View Post
You and no-one else can get more power than what the supply rails will allow.
re-biasing does NOT give more power.

Your 91Vdc is your power limiter.
Expect an absolute maximum of ~30Vac into a high impedance load.
Into a normal impedance load your maximum is likely to be in the range 20Vac to 25Vac.
i.e. 50W to 78W into 8r0
Andrew,
I'm not claiming to be able to do anything at all, that's why I'm on here asking questions that may be very silly.
Like I said, what I want is primarily to improve on what I've got with the aim to reduce hum and idle heat. Can it be done? Could I build the honeybadger, for instance, with the transformers I have?

Cheers,
Henrik
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Old 18th October 2013, 06:33 PM   #25
Mooly is offline Mooly  United Kingdom
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Hum is primarily down to the wiring scheme used, particularly grounds. It has to be right and any amp will hum if its grounds are not optimally configured.

Heat is down to biasing. Class B running say 100 milliamps per channel with one pair of transistors would generate around 4.5 watts of heat per transistor on your rails. So that's 18 watts for a stereo amp. With good heatsinking appropriate to the power rating of the amp (the 50 to 78 or so watts Andrew mentions) the amp would run quite cool under normal use.
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Old 18th October 2013, 06:35 PM   #26
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Henrik76 View Post
. Can it be done?
am I right that this will be your first DIY attempt ?

well, let me ask you, do you have another amp available ?

because these things may take time
and could also go wrong

maybe the best for you is a plug and play module, tested to work and ready for mounting

and then start on another DIY project

if you want more power, then its a whole different project than this

hmm
you have the right box for a bigger amp
but you have the wrong trafo
(I will not suggest series coupling and do the 1000watt thing )


but you probably need a new trafo with higher voltage

but maybe you amp is just not functioning like it should
2x 70watt can be quite powerful, if done right

ouch with the two trafos you have bridged/balanced would be possible
but if you can pull it off might be a different matter
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Old 18th October 2013, 09:19 PM   #27
jaycee is offline jaycee  United Kingdom
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Henrik, the type of power supply you describe is called a symmetric supply. As you have correctly deduced, it gives + and - voltage with respect to ground. This is pretty normal in most solid state amps.

With that case, heatsinks, and those power supply components.. you could build a very good amp of about 70W into 8 ohms. Look for an amp kit using at least 2 pairs of output transistors per channel, and preferably 3 if you plan driving it hard such as PA duty
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Old 19th October 2013, 11:28 AM   #28
bimo is offline bimo  Indonesia
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You must try current feedback amplifier that designed by Lazy Cat or Bonsai. I have built VSSA variant. This amplifier is sounding better than my voltage feedback amplifier which is more complex and better THD figure. Maybe because this amplifier have better slew rate and higher bandwidth.
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Old 19th October 2013, 11:48 AM   #29
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I'm agree with Bimo,
check this very simple one only use 6 transistor:
Click the image to open in full size.
I got that here PeeCeeBee
there are many other variant, but this is the easiest one to go

Regards
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Old 19th October 2013, 04:50 PM   #30
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Thank you all.
It will be my first DYI solid state amp, although I have built a few tube amps. I was hoping just to buy some kit off ebay, but there are so many designs and I have no idea which is better suited to the components I plan to re-use (transformers and case). The one that John Bali is referring to - is that available as a kit, with the board already printed and all components in a package? I don't mind loading the boards myself, but it would be great to be able to buy as a kit.

regards
Henrik
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