Hybrid Amp Build Direction/Recomendation

Hello,

I'm planning to build my next amp. I'm looking to do a pair of purchased Class D power modules with a tube input/buffer stage in one chassis as a stereo power amp that would require signal and volume control from an upstream preamplifier.

This amp in my case would receive signal from my preamp I built. It is an SRPP wired ECC802S with a gain of about 7. But I would like the amplifier to work well with commercially available preamplifiers.

My question is what would be a good tube buffer stage design for the pair of ICE Power 700AS1's I plan to use. I read the datasheet but am unsure of what kind of gain I need to obtain full output of the modules. Possibly a simple cathode follower would work even? I don't know?
The Ice Power module has an input impedance of 8K Ohms minimum.

Ice Power module datasheet is attached.

Thanks in advance
 

Attachments

  • ICEpower700AS1-X_Datasheet_1_6.pdf
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Last edited:

rayma

Member
2011-04-29 8:37 pm
An 8k load is difficult for tubes. The output capacitor would need to be 10uF
(unless it's in a feedback loop), and the output impedance should be 2k or less.
You likely have enough gain already, around 44dB total for line sources.
 
Last edited:

rayma

Member
2011-04-29 8:37 pm
I’m assuming the 10uf cap would be the coupling cap from the tube to the class d module.
What other specifics should I look for

Yes, the output capacitor. A warm up/turn off muting relay at the output would protect the power amplifier's input
from HV surges due to the large coupling capacitor.

Low small signal output impedance (under 2k) is necessary for low distortion into 8k loads. Also enough drive current
must be available. For full power, the amp needs around 2.3Vrms input, or about 3.2V peak. Into 8k, that is 0.4 mA
peak output current. The tube then should have a DC bias current of several times that, more than 2mA.
Look at a medium mu triode follower like 6SN7, 12AU7, etc.
 

rayma

Member
2011-04-29 8:37 pm
Attached is the schematic of my preamp. The only change I made is I used 2uf output coupling caps.

To start, add an output muting relay to make sure the amplifier isn't damaged by start up/power down HV transients.
The turn on delay should be 20-30 seconds, and the turn off should be immediate. Use a NC shorting relay to ground.

The next thing to add is a larger output C, try at least another 2uF in parallel with each, and see how you like it.
The small signal output impedance of this circuit is around 2.7k, so not too bad. It just may work well enough
like this, without adding more tubes.
 
To start, add an output muting relay to make sure the amplifier isn't damaged by start up/power down HV transients.
The turn on delay should be 20-30 seconds, and the turn off should be immediate. Use a NC shorting relay to ground.

This would mean that you have a relay normally short the signal+ input of the class D module to signal ground, and when a timer times out after 20s or so it disconnects the short to ground, correct?

Is it possible to have the signal relay just disconnect/ break the signal+ wire, or is it preferable to have it short to ground.