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Old 17th April 2003, 01:17 PM   #1
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Default Tube differential amp..not better than opamps

What gives..tubes instead of cheap opamp...no feedback instead of tonn's of feedback...tube rectified!...but this tube output stage actually sounds worse than the opamps..

I am shocked..

Everything is there..bass, treble..hard to say why it does not sound so good....it is like there is something missing...
maybe even order distortion?

Before I rip this stage up and revert to opamp..could anyone give me an opinion why this does not sound super de luxe..

filament is DC..

<img src=http://home.zonnet.nl/horneman/images/cd/cdpsu.gif border=0 alt="PF86 differential PSU"><br><br>
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Old 17th April 2003, 01:19 PM   #2
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Default Oops here is the schematic..

previous post was the psu
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Old 17th April 2003, 01:40 PM   #3
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I can't remember off hand the comfortable idle current range for your tube, but you might try varying it a bit. And to get lowest distortion, you can unbalance the plate resistors. Make the second half's resistor about 5-10% lower than the first half, or better yet, trim that resistor while monitoring the output for lowest distortion.
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Old 17th April 2003, 01:54 PM   #4
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Thanks Sy..

I will try altering the current a little..Did a little calculating...I can double the current without exceeding the anode dissapation! Will report back tomorrow.

Unfortunately I have no way of measuring for distortion other than my ears. Do you have a little background on why using different value resistors would lower distortion?

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Bas
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Old 17th April 2003, 02:20 PM   #5
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Well, one way to do it is to inject a signal and measure the AC voltage across each plate resistor, then unbalance the resistors until the voltages match. That will be pretty close to the distortion minimum.

Alternately, if you have a couple of high value (470K or greater) precision resistors (0.1%, say), tie one end of each resistor to a tube plate, then tie the other ends together and let that junction float. Inject a signal, then unbalance that resistor until the AC voltage at the precision resistors' junction is at a minimum.
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Old 17th April 2003, 02:36 PM   #6
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Bas, Sy

Sorry if this is off topic but can you use an LM3875 when doing a phono stage and if so, how would you switch in different RIAA curves?
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Old 17th April 2003, 02:46 PM   #7
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Why would you want to use a power amp for that function? If you want to make an IC-based phono preamp, there's the AD797.
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Old 17th April 2003, 02:50 PM   #8
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Sy

I didn't know if it was a dumb question but I thought a preamp's function was to amplify a source signal so that all the sources present the same level signal to the amps with the least distortion for widest bandwidth. Is there something different about an AD797 that makes it more suitable as a preamp device than an LM3875?
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Old 17th April 2003, 02:52 PM   #9
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Default Try This

Use triodes (suggest 6922's or 6CG7's) with 47K plate resistor. Current thru each tubes should be in the 3 to 5 ma range. Bias the grid relative to the cathodes using cathode dropping resistors.

Also be careful with your load - if necessary (and only if necessary) buffer the outputs.
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Old 17th April 2003, 02:54 PM   #10
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Hi,

Am no EE..but did a search on that chip..and saw that it is often used for the Gainclone..the gainclone being a power amp..it would be very unusual as a RIAA amp..

But I suppose it could be done if you use maybe a combination of passive RIAA and using the feedback loop..but I would be the wrong person to give advice...

I suppose your goal is to keep things as simple as possible and don't want a preamp and riaa amp..but a poweramp and riaa amp in one......sounds like a great idea if you just have a TT as source...the only real problem I suppose if having enough gain/input sensitivity..maybe you'd have to use a step up transformer from TT to gainglonce

Cheers,
Bas
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