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Imagine a 3-stage SET (input, driver, output tubes).
What kind of sonic differences would one expect between a direct coupled (all stages) and interstage, everything else being the same (just optinally adjusted for each topology).
My second question is if there is any benefit from an interstage between a input and a driver stage?
So basically, it's the question: a 3-stage DC vs 3-stage IT?
Hi Gordon,
You are asking excellent questions !! Here is MY opinion.
If you optimally adjust the topology of each amp type, the DC amp is NEVER a three stage amp, ever. It is a two stage amp, as in a Loftin-White, or variant thereof. Its been said about 30% of the DIY amps in Japan are L-Ws.
This gives the well executed two stage DC amp a huge advantage, because it can have a transparency, by eliminating the third stage, that NO three stage amp, no matter how well optimized, will have. Less, is more !! Go to three stages, in ANY amp topology, and you lose transparency, assuming optimum construction techniques are applied to both topologies.
DC versus Transformer coupled :
If one optimizes a DC amp and a transformer coupled amp, at the highest possible levels, the DC amp will win-out every time !! It hasn't the (1) bandwidth limitations that all transformers have, and it (2) does not have the phase shift problems an added transformer will inherently have.
The "rub" is, in a practical sense, very few people, if ANY, have ever truly optimized either type of topology, but the opinions I share herein is assuming one HAS totally optimized the amps' executions, in which case, I believe nothing will ever top a two stage DC SE amp.
YMMV, and that is fine with me. I understand.
Jeff Medwin
"If you optimally adjust the topology of each amp type, the DC amp is NEVER a three stage amp, ever."I have to ask, how many stages are in your CD player? How many stages in your phono pre?
Some say the best phono pre's have three stages. If I only have a two stage phono pre can I have a three stage power amp?
Please?
If I run my three stage power amp without a line stage preamp, is that the same as a two stage power amp being used with a one stage line amp?
Please tell me.
How many amplifier stages were use in recording the kick drum on your favorite song? How many stages for the lead vocal?
If there was one less for the vocal then a three stage would be "right" for the vocal but "wrong" for the kick drum?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/18/09
Tre',
I'll let you figure that out for yourself. I already have.
I own, but don't even use a CD player, I am phono only.
I don't discuss my phono system much, because it is unobtainium.
My phono preamplifier is fully direct coupled, so its pretty exciting to hear a DC RIAA EQ preamp into a DC amp, with a very good supply in both !
Jeff Medwin
Care to share your thoughts about the sonic effect of the slower rise time of the direct-coupled output stage (due to the need for a large cathode R), relative to other forms of coupling?Also note, you have outlined the negatives of TX-coupling relative to strengths of direct-coupling; there are other factors, which you perhaps feel are unimportant given your experimentations (fair enough), that are not discussed here.
Personally, direct-coupling is more appropriate for some tube combinations than others; same for TX-coupling and RC-coupling. As with most things, there is no absolute position of best - worst that can be applied to all situations. There are a variety of factors to be considered, which would shape one's decision making...
Disclaimer: I have not looked into the direct-coupling with stacked supplies approach. This may overcome of the challenges and limitations associated with direct-coupling, at a co$t and some additional complexity.
Cheers
Raymond"As long as we have any intention to be right, to show or teach the other person something, we should be wary. So long as words have the slightest ego attachment, they are dishonest."
Charlotte Joko Beck
Edits: 04/14/09 04/14/09
Go to three stages, in ANY amp topology, and you lose transparency, assuming optimum construction techniques are applied to both topologies.> >
Yes, but it isn't an even playing field. All the best DHTs have a mu of up to 9, occasionally 15 (1J6, 3a5). So your choice is:
a) 3 stage using DHTs like 10Y, 26, 01A, 71A
b) 2 stage using a high gain indirectly heated tube
This being the case, there's something to be said for either. I go for choice a) and I'd defend it sonically too. Others go for b) for obvious reasons. But it's a real choice.
andy
Consider the 841 or the 3C24; I've had good luck with both in 2-stage all-DHT amplifiers.
Hi Jim Dowdy 'Lama',
How would you characterize the sound of the 841 compared to other similar types like the 801a, the 10Y, or even non thoriated tungsten types? The 841 would seem to be a good choice as an input tube, although has similar Rp as a 12AX7. Thanks.
Regards,
fred
I've never really experimented much with the 801A, but can tell you that the 841 sounds very similar to the 10/10Y - just with more gain. Crisp and super-clean with nice frequency extension, it mates well with non-thoriated DHTs like the 2A3 and 300B.
As far as R(p) goes, I'd guesstimate that it's well under 30k as long as you draw some current through them. My op point: B+ of approx 385 vdc, R(k) of 750 ohms, drawing about 5 mA. Full frequency response with a 600 Hy Magnequest EXO-99 plate choke.
NOTE: I have used them successfully with a B+ as low as about 275 vdc, but prefer the higher voltages.
Hi Jim Dowdy Lama,Thank you for that golden info...
~30k Rp... I guess Mu would be lower than the published 30 too?
385 VDC, is that plate-to-cathode? Using 750 Ohms Rk, what bias voltage did you end up with? Needs a high L plate choke indeed for full range.
Regards,
fred
Edits: 04/17/09
I'm thinking the mu was pretty close to 30 utilizing the Magnequest EXO-99 plate choke...it was plenty of voltage swing for my 2-stage SE 2A3.385 vdc was the B+ measurement at the plate [to ground]. Sorry, I didn't record the bias voltage in my notes; my guesstimate is about -4 vdc.
Edits: 04/17/09
Hi Jim Dowdy Lama,
Thank you for the details. Yeah, the MQ plate choke would ‘preserve’ the gain, and maybe 75uF as cathode bypass.
It makes for an interesting input/driver tube for either a 2 or 3 stage amp. For a 3 stage, one example is 814 -> LC -> 10 (or maybe 45)-> IT -> 50....
Best regards,
fred
Hi Andy,
1) You are certainly not suggesting a three stage amp, are you?
2) You are suggesting a low mu DHT driving a DHT.
We have had this conversation before.
I suggest, if you know how to design an amp properly, the use of a DHT low mu driver tube will be a serious compromise, compared to a proper mu indirectly heated driver tube.
I have built both types. Its a shame you are on the other side of the pond, and that you employ low efficiency speakers, to reach your conclusions.
The low mu driver (DHT OR indirectly heated) will make for a dead amp on high efficiency speakers, particularly at lower listening levels, it will seriously lack "jump", unless pushed hard !!
Jeff Medwin
Hi Andy,
1) You are certainly not suggesting a three stage amp, are you?> > >
Yes, of course - exactly that.
We have had this conversation before. I suggest, if you know how to design an amp properly, the use of a DHT low mu driver tube will be a serious compromise, compared to a proper mu indirectly heated driver tube. > >
I suggest the opposite!
The low mu driver (DHT OR indirectly heated) will make for a dead amp on high efficiency speakers, particularly at lower listening levels, it will seriously lack "jump", unless pushed hard !! Jeff Medwin > >
I think it's a case of what we are looking for in a system. I'm a musician - not that that's special in some way but it does explain what I'm looking for, which is timbre and clarity. In an orchestra I want to hear everything and within that, instruments sounding as like as possible to their acoustic origins - cymbals like cymbals, oboes like oboes etc.
I get that with DHTs. That's why I use them. I think there's a significant gap in timbre between DHTs and indirectly heated tubes, that's why I don't mind a three stage amp in order to use them.
As we say - horses for courses!
Best, Andy
3 Stage DHT for me agree with what Andy has said
but Iam not as polite, anyone who uses idht over dht is
not listening to music, high mu idht drivers sound like crap.
Phono amps are the only place for them.
Rollin' tubes can certainly be rolled to our tatse. Our equipment choices are like tone controls...Certainly, our equipment choices are made to yield our favorite illusion of reality...
Direct Coupling all stages will be a complex circuit, that might require changes in circuit values as the tubes age. The speed of a DC ckt. can sound very impressive, very detailed. That can also yield a bright sound, sometimes too bright. If you are up to the task, try a Loftin White SET ckt. You probably only need 2 stages, one for gain, one for opt.
R-C coupling is simple, as is using an interstage tranny. Again, usually only one gain stage is needed, unless you do not use a preamp. One coupling cap in an amp is not a bad thing; even two or three can sound fine. IT coupling is, of course, my choice for PPT amps. Nothing has brought me closer to that real live sound, than using an IT for splitting the phase, in push-pull amps. However, this is an SE forum, so I will refrain...
Tubes seem to like transformer coils. While resistor loading works and can sound fine, the resurgence of IT and choke loading, even in SET amps, should be a wake-up call to, at least, try the various methodologies. In fact, try all the circuits you can think of. That way, you can choose YOUR favorite...
no contest. I'd opt for direct coupling. The advantage of an interstage is you can either increase or decrease voltages. but then this can also be designed into the various operating points of the tubes involved.
In theory I guess direct coupling should sound better, but there are some reasons for not using it.
The most obvious is that you're losing voltage on the driver stage since the cathode needs to be elevated. To some extent this can be compensated for by using choke loaded or transformer loaded or active loaded driver stages, but you may still be short of drive depending on what kind of output you want to drive.
This is just stating the obvious, so please feel free to ignore!
andy
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