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Old 25th January 2013, 05:31 PM   #1
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Default need suggestions for an AB amp project

I harvested a power transformer and a few misc other parts from a dead Denon AV receiver that a friend gave me, and I would like to build a simple power amp as project with the neighbor kid (we are presently building a fender champ clone at the moment).

If a suitable project exists with readily available PCBs, that would be ideal. This will hopefully be a "junk box leftovers" kind of project and I have rectifiers, IRF240s, IRF9240's etc left over parts from a Pass F4, mini-A, and F5 as well as a few heatsinks.

So, the transformer is from a Denon AV-1010 receiver with the following specs:
100Wx2 front
35Wx2 center
35Wx2 rear

The measured transformer voltages are:

main stereo 40-0-40 VAC

two other CT leads, 25-0-25 VAC and 18-0-18 VAC

and finally one more set of taps 0-4.5V

All voltages were measured unloaded.

I am assuming that this transformer is really only suitable for an AB amp, and that's fine, as I'd like to use heatsink leftovers that I have.

I'm thinking an AB mosfet based no-feedback amp but I am open to suggestions.
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Old 25th January 2013, 09:50 PM   #2
djoffe is offline djoffe  United States
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This recommendation is down a little different avenue, e.g. no reuse of the parts that you've shown, but perhaps provides an interesting alternative.

Akitika GT-101

Of course, if you have your heart set on the resurrection build, that can be fun too. It's just that the 40-0-40 is a little high for the average chip amp, which would be an easy way to go...
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Old 25th January 2013, 10:58 PM   #3
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Quote:
Originally Posted by boywonder View Post
main stereo 40-0-40 VAC
two other CT leads, 25-0-25 VAC and 18-0-18 VAC
and finally one more set of taps 0-4.5V
Some possibilities:
40,0,40vac Honey Badger stereo main channels
40,0,40vac alternative, Honey Badger subwoofer channel (put 220u in for C4)
25,0,25vac BPA300 LM3886, or bridged Honey Badger, or bridged+parallel TDA7293 monophonic subwoofer channel
25,0,25vac alternative, TDA7293 surround channels.
25,0,25vac alternative2, Circlophone or SSA or MyRef-FE main channels.
18,0,18vac LM1875 or TDA7295 surround channels (with tiny input caps and small-ish output caps for ~150hz)
18,0,18vac alternative: 15vdc regulators running a preamp.
4.5vac single, DAC, or led's
4.5vac single, alternative, voltmulti+regulator for remote control kit.

P.S. With 4 bridge rectifiers all yelling at one transformer, I'm assuming that you want RC snubbers per each transformer secondary (if one is expecting hi-fi).
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Old 25th January 2013, 11:17 PM   #4
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Quote:
Originally Posted by danielwritesbac View Post
Some possibilities:
40,0,40vac Honey Badger stereo main channels
40,0,40vac alternative, Honey Badger subwoofer channel (put 220u in for C4)
25,0,25vac BPA300 LM3886, or bridged Honey Badger, or bridged+parallel TDA7293 monophonic subwoofer channel
25,0,25vac alternative, TDA7293 surround channels.
25,0,25vac alternative2, Circlophone or SSA or MyRef-FE main channels.
18,0,18vac LM1875 or TDA7295 surround channels (with tiny input caps and small-ish output caps for ~150hz)
18,0,18vac alternative: 15vdc regulators running a preamp.
4.5vac single, DAC, or led's
4.5vac single, alternative, voltmulti+regulator for remote control kit.

P.S. With 4 bridge rectifiers all yelling at one transformer, I'm assuming that you want RC snubbers per each transformer secondary (if one is expecting hi-fi).
How much 40-0-40 VAC current do I need for the Honey Badger? I see this board in the store so that's appealing.....

I like the idea of using the 25-0-25 VAC output for another lower power stereo output......I'll have to research MyRef-FE and TDA7293.

Can you provide more details on the RC snubbers?

I usually hang out in either the tubes forum or the Pass Labs forum so these projects are all rather new to me.

Thanks!
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Old 26th January 2013, 08:28 AM   #5
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Quote:
Originally Posted by boywonder View Post
How much 40-0-40 VAC current do I need for the Honey Badger? I see this board in the store so that's appealing.....
For a stereo pair, approximately 7.5a, but it could run with more or less.
Quote:
Originally Posted by boywonder View Post
I like the idea of using the 25-0-25 VAC output for another lower power stereo output......I'll have to research MyRef-FE and TDA7293.
MyRef-FE has good performance for an LM3886 build and is tuned not to shout, much unlike common gainclone kits. I've made publications for TDA7293 and while the tone is similar, the MyRef's resolution might be higher, but that topic depends on power circuit, mainly.
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Originally Posted by boywonder View Post
Can you provide more details on the RC snubbers?
Installation: Well, if you don't have a scope handy, try a 2u2 polyester in series to a 50 ohm multi-turn variable resistor to create an adjustable snubber (credits to Keantoken for creating a "kitchen table" method, by request). This RC (a tone control) across transformer's secondaries allows you to muffle the noise, not the audio. The RC is installed directly at the bridge rectifier, for example from the "~" to the "~" of the KBPC1004.

Compatibility: A power circuit that combines snubbing transformer secondaries with CRC filtering at the power supply is compatible with a wide range of ordinary to botique bridge rectifiers. Snubbing the transformer secondaries has a dual purpose--confine bridge rectifier noise to the bridge rectifier locale, and "waste some noise" into the RC's resistor.

Snubber theory: The cap of an RC snubber prevents wasting good power; however, the resistor of a snubber does the majority of the noise removing task.

Adjustment of the dial may or may not have some of the following:
Clearer and/or higher resolution audio.
A more level sounding (less peakish mids&treble harmonic) response.
A somewhat cooler amplifier heatsink.
*These are practical indications that a noise filter has worked. Scope use would be good; however, the filtering does have some practical benefits that can be used for approximation. If the filter seems non-working, you can change the cap size and re-try. Snubbed transformer secondaries are a normal (and perhaps expected) part of a hi-fi amplifier.

P.S.
If you've done the above first, that filtering has reduced the need of DC side snubbers so that you can use DC side RC cap values so tiny that they could not dull the audio. Yay! It is possible/probable that ordinary CRC power supply would have completed the DC side job and then DC side snubbers aren't required at the power board.
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Last edited by danielwritesbac; 26th January 2013 at 08:57 AM.
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Old 26th January 2013, 03:11 PM   #6
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Thanks Daniel, I do have a scope, so how do I dial in the snubber with a scope and func generator?

Are there any lower powered AB projects to recommend with good performance? Say 25-60W or so? We don't need much horsepower.

I also have a few BrianGT PS boards and Peter Daniels PS boards.

Last edited by boywonder; 26th January 2013 at 03:25 PM.
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Old 26th January 2013, 06:00 PM   #7
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Try this one
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Old 26th January 2013, 06:04 PM   #8
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Quote:
Originally Posted by boywonder View Post
Thanks Daniel, I do have a scope, so how do I dial in the snubber with a scope and func generator?
I don't know, but maybe this will help: PRACTICAL SNUBBER AND TERMINATION DESIGN
Quote:
Originally Posted by boywonder View Post
Are there any lower powered AB projects to recommend with good performance? Say 25-60W or so? We don't need much horsepower.
SSA and Circlophone
A parallel LM1875 may also be interesting.
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Old 29th January 2013, 12:20 AM   #9
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Boywonder, if you can probe the trafo magnetic field with an inductor, or measure the charging currents using a .1R resistor or such, you can tune the snubber by hand while watching the scope. I found the scope was good for finding the general value, but finer tuning should be done by ear near that point. Also try turning the trimmer all the way to 0; that doesn't snub anything but may sound good because of the decoupling effect (I found I preferred the other way though). For a full treatment of trafo resonances more snubbers are needed, but this one will take care of most of the problem. I haven't quite worked out how to adjust the others since it's a nonlinear problem.

I don't know whether the effect of the snubber will be different for toroidal and non-toroidal trafos. Any resonance around the trafo is radiated magnetically, so toroids may not need it as much as EI trafos.
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Old 29th January 2013, 06:29 AM   #10
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The reason that I brought it up is that his transformer has a lot of secondary windings and therefore may have a gang of bridge rectifiers all yelling at one transformer. Oh no, a big noise generator! So, I believe that if one wants hi-fi from that system, all of the bridge rectifiers' racket must be confined to the bridge rectifiers' individual locales, in order to Block an each to all pollution situation.

Even after snubbing the secondaries, I'm fairly sure that any small signal purposed DC supply will need high performance K-Multi followed by regulators before it can successfully power high fidelity small signal. For good performance, a discrete "regulated front end" power amplifier (hopefully with paralleled output devices) seems to be the most straightforward solution if one is also preparing to drive modern speakers.

It seems to me that monoblocs with 1 bridge rectifier per 1 transformer might make some matters a bit easier?
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