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Old 1st September 2003, 11:33 AM   #1
GAK is offline GAK
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Question srpp or m-follower?

Hi

I'm planning to build a preamp.I'm between an SRPP and m-follower circuit.
I know the advantages for the follower circuit from Morgan Jones' book that uses the E88CC for the upper stage and the ECC82 for the lower stage.
I find it very nice to build it but what about the SRPP circuits?
I know that they're good,too.
And Morgan doesn't have any information about these circuits.
I have some other books with schems on SRPPs circuits(with ECC82,6N1P,ECC83...) but with not so much information.

Which is the best circuit?And why?
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Old 1st September 2003, 11:44 AM   #2
dhaen is offline dhaen  Europe
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Hi,

IMO the mu follower beats the SRPP. It's ouput impedance is lower, and the lower valve runs under optimal conditions.
The only dissadvantage of the mu follower is the high voltage supply that is required.
Maybe someone will come and claim the SRPP's attributes...

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Old 1st September 2003, 11:50 AM   #3
AKSA is offline AKSA  Australia
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The Mu follower lower tube indeed runs under optimal conditions, enabling much higher gain, almost mu in fact, for the stage.

The SRPP loads the lower tube right down, typically returning a gain around 40% of the lower mu (unbypassed) and 60% (bypassed). This certainly indicates a heavy plate loading.

In my opinion, and I'm only one guy, comparisons between the two topologies, all other things being equal (same tube), the SRPP has better sonics, more musical.

I suspect this is because the distortion spectrum, though much larger for the SRPP, is more favorable somehow. This to me would indicate perhaps more H2 and H3 in the lower gain SRPP.

Cheers,

Hugh
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Old 1st September 2003, 12:28 PM   #4
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Hi,

Quote:
I'm planning to build a preamp.I'm between an SRPP and m-follower circuit.
If it's for a linestage only I'd definetely go for the SRPP and leave the lower cathode unbypassed.

Quote:
I suspect this is because the distortion spectrum, though much larger for the SRPP, is more favorable somehow. This to me would indicate perhaps more H2 and H3 in the lower gain SRPP.
I agree...

In between the classic SRPP and the -follower there's also the "optimised" SRPP.

Cheers,
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Old 1st September 2003, 12:57 PM   #5
dhaen is offline dhaen  Europe
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Some SRPP info from Lynn Olson's site:
http://www.aloha-audio.com/library/FindingCG.html [url]

Click the image to open in full size.

Cheers,
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Old 1st September 2003, 02:26 PM   #6
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Default Mu

Quote:
Originally posted by GAK
Hi

I'm planning to build a preamp.I'm between an SRPP and m-follower circuit.
I know the advantages for the follower circuit from Morgan Jones' book that uses the E88CC for the upper stage and the ECC82 for the lower stage.
I find it very nice to build it but what about the SRPP circuits?
I know that they're good,too.
And Morgan doesn't have any information about these circuits.
I have some other books with schems on SRPPs circuits(with ECC82,6N1P,ECC83...) but with not so much information.

Which is the best circuit?And why?

Mu. For all the tech reason's Dhaen outlined below, as well as the fact it sounds a whole lot better. Every SRPP I've ever heard, has sucked. Then again you might like it's flavour.

Personally, I don't bother with tubes for the upper half anymore. There are a number of SS solutions that measure and sound better providing you're not sand-o-phobic.

Gary Pimm has some excellent CCS (constant current sources) that work in mu stages beautifully well. This one would be my suggestion, and only takes a B+ about 50V above the tubes anode voltage to work in a linestage.

Also look at his schematics page for more variations incl some hybrid tube/fet designs and the detail on the theory and how to set them up.

Another variation using depletion mode devices can be found at Bas' mosfet page
This has the advantage of not needing the batteries, but the batteries lasted for months in mine and I could detect no change in voltage; according to Gary the battery life shouldn't be much shorter than their shelf life. I've built both and am using the DN2540 cascode version in my "linestage" at the moment. It's very, very good sounding, so much so that I can't be bothered playing with anything else. The tube I'm using is the 12B4A, which is excellent sounding, has about the right amount of gain, and is cheap ($US3) and available. It does like a DC heater supply in a linestage though. I'd suggest giving it a try at Va-k ~ 120V, Ia = 30mA and a B+ around 200V.

Everything You Ever Wanted To Know About SRPP But Were Afraid To Ask
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Old 1st September 2003, 10:12 PM   #7
AKSA is offline AKSA  Australia
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I have built and road tested SRPPs, mu followers with tube and mosfet uppers, and I strongly disagree with the last poster.

It is generally a fact that in this game we rarely compare apples and apples; controlled experiments with identical operating points, tubes, and output loading are rare. Consequently preferences one way or the other are often based on false premises.

As long as the distortion is not high order - viz 5th and higher - a THD up to about 1% is close to irrelevant to the sonics. But you will notice big differences in sound between different tube operating points and topologies.

You are building something which you hope will sound good, but if it measures well it is merely an intellectual bonus. Math and measure in this game has done a lot of harm over the years, although it's sold a lot of very impressive instruments for measuring distortion...... We are organic lifeforms, not machines, and our ears do not perceive the same things as measuring instruments. We are more subtle, and go for things like 'ambience', 'pace, rhythym and timing', and 'tonal balance'. These are well nigh impossible to measure, but they are highly relevant for anyone into good sound.

If you design commercially you soon come to realize that when assessing gear people put great stock in how in sounds. This is common sense, and shows that when the chips are down, and money is involved, their selection criteria are reasonably rational. If they build their own, however, they often go back to measurements, particularly THD and frequency response, and while these are good places to start, they are only a portion of the art. Any good design still has to be 'voiced'.

You'd like to build a two tube series amplifier (TTSA), but the best configuration, in my view, and after listening to a heap of them, is the conventional plate loaded triode. Pick a nice grunty triode like a 6BX7, or even a pentode like an EL84 wired as triode, and go for it. You'll get good gain, reasonable output impedance, and wonderful sound, and it will be simpler. The difficult part then becomes the power supply, but then power supply is always difficult.....

Cheers,

Hugh
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Old 1st September 2003, 10:37 PM   #8
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Hi,

Quote:
You'd like to build a two tube series amplifier (TTSA), but the best configuration, in my view, and after listening to a heap of them, is the conventional plate loaded triode. Pick a nice grunty triode like a 6BX7, or even a pentode like an EL84 wired as triode, and go for it. You'll get good gain, reasonable output impedance, and wonderful sound, and it will be simpler. The difficult part then becomes the power supply, but then power supply is always difficult.....
Knowing Brett's design philosophy I think he would agree with your philosophy; he just takes a different road to achieve his goal.
I feel alot of the sonic fingerprints of the SS CCSs are camouflaged by the use of massive amounts of iron; IT xformers etc.
I suppose Brett won't agree and i understand his POV.

Brett, I hope you don't mind this?

While I'm in total agreement with you Hugh, I avoid sand devices as much as I can except for where I feel it either doesn't matter for the sound of things or where I feel that using valves is too much of a hassle...

As far as I'm concerned that usually means heater supplies and other low voltage apps.

When it comes to PS designs, no SS HT regs please: they either die on you or/and make the whole circuit sound like sand.

So for me, it's valved regs ( I don't mind silicon rectifier diodes in this context provided they're snubbered or Schottkys) shunt or series depending.

Sidebar: Any news on the Balanced Pre front? I gave up posting there but have some ideas...

One thing I've always hates was cathode bypass caps...with the advent of BG caps I may revise that prejudice.

If there's one thing we seem to have in common it's that we all seem to like decent current running through outr bottles.

Cheers,
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Old 1st September 2003, 11:05 PM   #9
AKSA is offline AKSA  Australia
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Hi Frank,

Thank you for your post; you write interestingly of SS CCS.

I recall reading an article in Glass Audio (I think!) some years back where someone compared distortion spectra with loading on a 6SN7 triode.

As you increased the loading (viz, reduced the ohms of the plate load resistor), the distortion spectrum begins to increase, mostly generating H2 and H3. With infinite plate resistance (sometimes called a CCS) the Av of the stage almost equalled the mu. The output impedance was then the rp of the tube, and the distortion was extremely low. Interestingly, of course, the Zout was actually lower with a resistive loading.

But CCS loading didn't sound quite as luscious as a resistive plate load chosen around 2.5rp.

This makes no comment on a transformer or choke loaded triode, I should add.

A little understood problem of the CCS loading is noise. When you hold a reference voltage on the base of a CCS transistor, any noise at all is amplified, typically by 60dB and more, to the collector. This means that even microvolts at the base show up as millivolts of grunge at the collector. Consequently, the voltage reference had better be very quiet; I use a LED, but even LEDs create noise. There are tricks to rid this noise further, a good one is splitting the current feed resistor from the LED to ground and running a bypassed lytic from rail to the midpoint of this resistor. However, noise still remains. Only the very best caps at this decoupling point will scotch most of it; a Black Gate here works well, but I've yet to fully explore the possibilities, I know the ZL series from Rubycon are good too.

But the point is that this CCS grunge, extending right up into the supersonic range, is significant and damages the music, chiefly by robbing resolution and greying over the 'black between the notes'.

Thus, I'm not in favor of CCS use with tubes, but I do admit that power supply design for plate loaded triodes is not trivial, and very difficult, in fact.

Cheers,

Hugh
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Old 2nd September 2003, 12:46 AM   #10
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Quote:
Originally posted by fdegrove
Knowing Brett's design philosophy I think he would agree with your philosophy; he just takes a different road to achieve his goal.
I feel alot of the sonic fingerprints of the SS CCSs are camouflaged by the use of massive amounts of iron; IT xformers etc.
I suppose Brett won't agree and i understand his POV.
If you're referring to my system, I have only two peices of iron in the signal path, an S&B TVC and the poweramp OPT, and two coupling caps in 4 stages from cart to speaker.

And, no, I don't agree with your analysis Frank.

As for the noise issue raised, I don't hear it on a system that's very sensitive to noise (106dB/1W speakers), so to me it's moot.

Harmonic spectra; Whilst what Hugh says about the spectra is true, the IMD will be lower with a CCS-mu stage, and having speakers with very low IMD, I hear that clearly and it annoys me considerably more than THD.

I've tried enough circuits and variations of them over time that I trust my ears on what I prefer, and to my ears SRPP sucks. I'm also pretty good at telling quickly what has potential and play with op-points etc to see if I can improve it to my ears. Hell, two different tubes from the same box often sound different unless you have a curve tracer and are able to match them closely. Measurement was always secondary, but interesting nonetheless.

A mu-stage will generally have a lower Zout than any sensible R loaded CC stage, same tube and op-point. An EL84 is a great little tube triode loaded, but it suffers from a heap of input capacitance which needs to be driven (>200pF) and are generally a bit microphonic in linestage apps IME. Lynn Olson commented it's like a 6SN7 with grunt (paraphrasing), and I'd agree.

As Hugh's 5th paragraph in the post immediately after my previous one is a barely concealed cheap swipe at me, if we were face to face, I'd be more blunt in my response, but basically, get stuffed. I've spent a long time in the electronics field designing and building commercial products and I understand the tradeoffs inherent in business. Because you design commercially has little to do with the ultimate performance of your product and far more to do with marketing, sustainability (esp to availability of NOS tubes), size, shipping weight, legal liability, heat, power consumption, hype, follow-the-leader etc. As for people buying what sounds best, bollocks, they buy what they beleive sounds good, or what they're told sounds good, as most haven't heard anything better. For example, your business was built on amplifier modules that few people ever get to hear in their system before they buy them, so once a lot of time money and effort is expended in doing so, most people are going to convince themselves it sounds great even if it sounds like a sack of shite (NB: I've never heard an AKSA amp, so I'm not commenting on the sonics of these amps). This also happens with people who buy manufactured products, especially when they spend a lot of money on them.

All that said, I do wonder at the topology of the AKSA tube amp stages. Low loaded CC or SRPP perhaps?

This comment in the post before was almost funny; 'black between the notes', because this is one of the main criteria I use (and have for years) and is not one of my systems lesser points.

Last point. Hugh, so far you've tried to skewer me with all sorts of presumption about the topologies of the circuits I use, what I have/haven't tried, how I test etc, and you've been nailed every time, so just give it up. It's boring the eff out of me. We obviously disagree and because you're "an audio manufacturer" I'm not the least bit impressed and I'm definitely not one of the mass of groupies that pervade the hobby. Then again, I don't have to run around and justify my design decisions, as I have few of the limitations you do, and I'd rather my gear performed like a McLaren F1 (road) than an R16.
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