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Old 18th July 2008, 07:33 PM   #1
spsfahy is offline spsfahy  United States
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Default Impedance problems

I'm very pleased with the sound of each of the amps individually. The power amp sounded great with the Bride of Zen as it's preamp. I've listened to the Aikido through other power amps and it sounds fantastic.

When I connect the two together the distortion is beyond belief. My knowledge is limited, I'm a build and hope it sounds good type person. It looks like an impedance problem. The only circuit changes I've made in that area is the removal of the input capacitor on the power amp. In or out it sounded horrible. I've left it out.

If it's a buffer I need, the only place I have room is in the power amp, so it would have to be solid state.

Preamp:
http://www.tubecad.com/2004/AikidoAmp3.gif

Power Amp: http://www.diyaudio.com/forums/attac...57560308:xeye:
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Old 18th July 2008, 08:33 PM   #2
gain is offline gain  United States
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sounds like the input to the amp may be excessively loading the output of the preamp, causing nonlinearities. you could breadboard up a simple op-amp circuit to put between the preamp and the power amp and see if that fixes the problem. no need to get fancy as this is just for testing ... use a 741 and two resistors if you want. if the distortion noticeably improves, then you can build a better version with tlo74/84 or whatever your favorite op-amp is.

also i would recommend you put the input coupling cap back in to protect the amp from dc offsets at its input.

regards.
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Old 18th July 2008, 08:47 PM   #3
lineup is offline lineup  Sweden
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The output cap in Aikido tube amp you posted is
0.47 uF
This indicates that it is intended to drive high impedances.
I would say >= 100 kohm.

Many power amplifiers has got input impedance lower than this.
22 kohm is not unusual .. even sometimes 10 kohm

This means maybe 5-10 times more load, than Aikido was meant for.

To build a hifi buffer with an Op-Amp, like suggested, is not very difficult.
There are 1000 of such gain x1 circuits in internet and in this forum.
Of course such an opamp circuit need a good dual regulated supply.
For example +- 15 VDC. Or +- 12 VDC.
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Old 18th July 2008, 08:55 PM   #4
gain is offline gain  United States
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if you didn't want to go through the trouble of building a bipolar supply you could build a single ended unity gain amp with discrete components instead of an op-amp. might even be simpler/cheaper for your application, when you factor in the additional componets required for a bipolar supply.
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Old 18th July 2008, 11:00 PM   #5
spsfahy is offline spsfahy  United States
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I had thought of putting an op amp in there but I was afraid that the end result would limit the sound quality to no better that what the op amp would produce. I'm very interested in going with the "single ended unity gain amp with discrete components" it's more in line with the circuitry in place now. Do you have any ideas as far as a safe circuit to match the impedances without introducing much of a change in sound. I know any time you add something it's noticeable, I just want to keep it at a minimum.
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Old 18th July 2008, 11:18 PM   #6
lineup is offline lineup  Sweden
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hi.

Using only 2 N-JFET, you can get close to a perfect Buffer.
It will work in Class A.
It is my favourite !!!

Quote:
Here is nice & easy Buffer, using 2 N-JFETs.

You can find similar Buffers at the the web, using 2 JFETs.
One upper and one lower of same sort, for Current Source.
Also in our forum, you can find such circuits.
After some searching


Topic:
Audio buffer amplifier
Post:
http://www.diyaudio.com/forums/showt...70#post1539670
Schematic:
http://www.diyaudio.com/forums/attac...amp=1213563029


Regars, Lineup
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Old 19th July 2008, 05:42 AM   #7
spsfahy is offline spsfahy  United States
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Thanks to both of you folks for your help. I learned something new about buffers. I think I'll go with a bipolar circuit I found on the post link because the input impedance of the power amp is very low. Plus I already have a power source. The main thing is that I feel as if I will not lower the overall quality.

Best regards,

Sean
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