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Old 1st July 2007, 03:01 AM   #1
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Default SS or tube voltage amplifying stages in an amp with tube output?

I thought I'd start a new thread on this, rather than hijack another one. This topic interests me and I haven't seen another thread that really addresses it.

IMHO, the main advantage of a tube amp over an SS amp is the ability of a tube output stage to clip relatively benignly, especially if care is taken in the design to ensure that this will be the case. Most SS amps clip suddenly and sound very harsh at overload. This can give the impression that a 25 watt tube amp seems more powerful than a 50 watt SS amp, especially if there are significant transients in the signal.

However, I don't think tubes really offer any advantage over SS in an amp's front end or preamp, assuming good design and use of appropriate devices. I haven't experimented much in this area, except to say that I've used signal sources, such as CD players and radio receivers, which incorporate SS circuitry.

So, my question is: does it really make any difference whether voltage amplifying stages are built using tubes or SS devices, in your experience?
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Old 1st July 2007, 03:30 AM   #2
SY is offline SY  United States
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A tube input/driver stage will result in an amp with better linearity than the same amp with ss voltage amplification. OTOH, for follower service driving output stages, silicon rules.
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Old 1st July 2007, 04:19 AM   #3
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Hi RayMoth , Hi SY ,

In my humble opinion , the best topology ( at least for me ) is :

First stage ( voltage amplifier ) A SS Operational Amplifier
of last generation .

Phase Inverter A tube Concertina phase inverter or long
tail pair , with CCS .

Driver Stage Tubes
Driver Stage for Class AB2 Tubes as Cathode Followers

Output Stage Tubes in push-pull way .

What do you think about that ??

Regards ,

Carlos
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Old 1st July 2007, 12:52 PM   #4
SY is offline SY  United States
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Since it's easy to get a tube that will swing the 30-40V needed by an output stage at stunningly good linearity before applying feedback, I'm unclear on the desirability of using an opamp in that spot.

In my own amps, cathode followers driving the output stage worked no better (and in most cases, quite a bit worse) than MOSFET source followers.

Lest I sound too much like the Anti-Carlos, I'm in full agreement that in modern amps, either a split load or long tail inverter will give essentially perfect balance and there's no need to use the inferior circuits of the '40s and '50s. And with the right output tubes (e.g., 6L6 types), push-pull AB2 is very much the way to go.
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Old 1st July 2007, 06:31 PM   #5
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Default Re: SS or tube voltage amplifying stages in an amp with tube output?

Quote:
Originally posted by ray_moth
However, I don't think tubes really offer any advantage over SS in an amp's front end or preamp, assuming good design and use of appropriate devices. I haven't experimented much in this area, except to say that I've used signal sources, such as CD players and radio receivers, which incorporate SS circuitry.

So, my question is: does it really make any difference whether voltage amplifying stages are built using tubes or SS devices, in your experience?
The main advantage of hollow state over solid state is the greater linearity and the freedom from distortion mechanisms such as parametric frequency multiplication. One of the worst VTs I've ever found, the 12AV7, is considerably better than any small signal BJT.

If you want to build a great amp, the place to start is with the open loop performance. If you do the open loop design right, then you don't need enormous amounts of gNFB to sweep your design flaws under the carpet -- and along with them, lots of the musical detail as well. A solid state front end is going to have worse distortion, and require that much more gNFB to clean up the SS messes. If you're doing a pure solid state design, that is of no consequence since BJTs like lots of gNFB.

You could make your solid state front end as a separate unit, give it lots of open loop gain and gNFB, and use it as a line stage. After all, the music's already been through a whole bunch of SS from the studio mike to the CD/tape/vinyl, so what's a few more?

Within the VT amp itself, the only place for SS in the signal path is a MOSFET source follower used as a Class A2 or Class AB2 grid driver. Source followers are pretty benign, and can slam current into a grid while presenting it with a very LoZ better than any hollow state device.
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Old 1st July 2007, 08:58 PM   #6
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If you're going to accept a valve output stage (and almost certainly accompanying transformer), why sully it with devices that need buckets of NFB to make them linear? It makes sense to apply buckets of feedback in an all-transistor amplifier because the lack of output transformer makes all that feedback viable. Now if you were suggesting valves to drive a transistor output stage...
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Old 1st July 2007, 09:03 PM   #7
Hartono is offline Hartono  Indonesia
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valve for power amp front end ?

edit : I mean valve for solid state power amp front end
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Old 1st July 2007, 11:14 PM   #8
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I would rather use transistors as follower type output stages than to use them anywhere for voltage amplification.... Yuck!
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Old 1st July 2007, 11:43 PM   #9
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I've been known for the annoying habit of going back to first principals when designing anything.

The triode tube is the most linear amplifying device ever invented. Need we say any more?

Cheers,
Ian
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Old 2nd July 2007, 12:11 AM   #10
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I used a transistor in front of a tube in microphone preamp before I found a relatively cheap source of microphone transformers with needed ratio.

Click the image to open in full size.
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