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Old 4th October 2011, 06:26 PM   #1
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Default any suggestions for a mid-amp transformer for a hybrid design... like this circuit...

see page 4 of this service manual...

http://valvetronix.net/docs/AD120VT_...l_Complete.pdf

I'm wanting to experiment with a similar idea... Namely a 'mini' tube push pull output stage that goes through a transformer and then gets some beef from a solid state power stage...

Fotr anyone who's not familiar the valvetronix are largely DSP guitar amps that use a 12ax7 as a fake power amp in push-pull. I don't know if the tubes actually make much difference used like this... Hence my wish to experiment.

Any idea where I can get a Xfmr that would work in this context? I confess I don't know much about Xfmrs and their uses, aside from in a straight forward power or o/p sense...
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Old 4th October 2011, 06:52 PM   #2
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For small signal and mids modemtransformers could do $2 !

They have nothing below 300hz and reach up to 5-7khz.

Otherwise if you donīt need the lows any line transformer with right ratio will do.
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Old 4th October 2011, 07:29 PM   #3
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Thanks for your reply...

Sorry guys, I wasn't very clear...

By mid-amp I meant not at the 'output end' or the power supply, rather than an amp for amplifying mid-range frequencies... I guess interstage might have been a better way of putting it?

I'll edit the OP slightly...

EDIT:
I can't edit the OP... Oh well...

Last edited by bassetrox; 4th October 2011 at 07:38 PM. Reason: Inability to edit!
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Old 4th October 2011, 07:49 PM   #4
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The whole idea is pointless.
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Old 4th October 2011, 08:07 PM   #5
kevinkr is offline kevinkr  United States
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Actually Fisher built hybrid tube solid-state power amps for one of the their mid 1960s high end stereo consoles. (Perpetuum Ebner changer, magnetic cart, and a very good FM stereo tuner, 3 way acoustic suspension speaker system per channel) Power was about 50Wrms per channel.

The driver interstage had a pushpull primary, and two separate secondaries driving a quasi-complementary output stage - the transformer did the phase splitting. The whole thing sounded quite good, but definitely was not your typical console either.

The output stage would of course be unity gain in this topology so step down ratio of the transformer would have to be chosen such that the output voltage was sufficient to do the job given the driver tube chosen. Possibly that one of the smaller Lundahl PP/SE OPTs could be used in such an application with some careful thought and reconfiguration.. (Either SE or PP drive - most have 4 secondaries that can be wired in a variety of ways)

You might also consider a PP plate to line transformer if you do not mind having to use a power stage with some gain.
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Old 4th October 2011, 09:45 PM   #6
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Quote:
Originally Posted by kevinkr View Post
Actually Fisher built hybrid tube solid-state power amps for one of the their mid 1960s high end stereo consoles. (Perpetuum Ebner changer, magnetic cart, and a very good FM stereo tuner, 3 way acoustic suspension speaker system per channel) Power was about 50Wrms per channel.

The driver interstage had a pushpull primary, and two separate secondaries driving a quasi-complementary output stage - the transformer did the phase splitting. The whole thing sounded quite good, but definitely was not your typical console either.

The output stage would of course be unity gain in this topology so step down ratio of the transformer would have to be chosen such that the output voltage was sufficient to do the job given the driver tube chosen. Possibly that one of the smaller Lundahl PP/SE OPTs could be used in such an application with some careful thought and reconfiguration.. (Either SE or PP drive - most have 4 secondaries that can be wired in a variety of ways)

You might also consider a PP plate to line transformer if you do not mind having to use a power stage with some gain.
Thanks, that's really interesting. I'll see if I can do some research on those systems...

Yeah, I'm thinking unity gain for the tubed section and then maybe a 20-30W solid stage after that, maybe just using a TDA 'power-amp-in-a-chip'. Like I say, I'm doing it for the sake of a proof (or denial!) of concept.

Thanks for your help.
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Old 4th October 2011, 10:39 PM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by bassetrox View Post
Thanks, that's really interesting. I'll see if I can do some research on those systems...

Yeah, I'm thinking unity gain for the tubed section and then maybe a 20-30W solid stage after that, maybe just using a TDA 'power-amp-in-a-chip'.
Don't do unity gain. It will sound like a short length of wire and be a waste of time. Distortion, by definition is non-linear gain. So no gain, not change in the sound.

OK you can do unity gain but only if you have again of 10 to 60 and then use a voltage divider to knock it back down. That kind of gain is not a true "unity gain amp".

There are several transformers that would work but the Hammonmd 125A is perfect. Also it can re wired for many different inpedances ratios so it is a good experimenter's transformer. I would set it up at 25,000 ohms and drive it with a 12AU7. The 4 ohm output would be reasonable for driving a chip amp but certainly resistor network between the 125A and chip amp is required


But for your experiment you can get the same reslt using any small guitar amp, then feeding a resistive attenuator and then a HiFi amp and then a guitar speaker, no need to build anything
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Old 5th October 2011, 12:53 AM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by bassetrox View Post
Thanks, that's really interesting. I'll see if I can do some research on those systems...

Yeah, I'm thinking unity gain for the tubed section and then maybe a 20-30W solid stage after that, maybe just using a TDA 'power-amp-in-a-chip'. Like I say, I'm doing it for the sake of a proof (or denial!) of concept.

Thanks for your help.
No, put all the gain in the tube section and make the solid state section unity gain or close to it.. If your aim is effects this should give you the distortion characteristics of tubes with solid state drive capability, if that is not your goal the character will be more tube like than the other way round still with the solid state drive capabilities..

Might help if you explain the intended purpose. MI or HIFI?
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Old 5th October 2011, 08:27 AM   #9
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Don't agree.
A classic valve guitar amp is normally run pentode mode with no feedback.
This allows the characteristics of the amp to interact with the various characteristics of the speaker and it's cabinet, and this is where the 'magic' of guitar sound is created. If you put a solid state amp in between with all it's negative feedback and massive damping factor, then you lose everything. that's why I said it was pointless earlier.
It may be Ok in a HiFi amp but not for guitar.
Regards
Henry
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Old 5th October 2011, 09:47 AM   #10
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Going the chipamp route you have no control over NFB.

Discrete and well design might work.

A chipamp is designed for good "HI-FI" and guitar amps are real bad in hifi terms.
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