[FullRangeMan]

Dear Tube fans:
These are the Mystere(made in China) CA-11 and CA-21 preamps from Durob Audio (Holland), Iam salivating on these preamps cause they use my beloved 6SN7 input tube.
After close looking in the site, I find just three problems in these nice brothers:
1) They have no motorized remote volume control (price is low), but they have a hand made resistive potentiometer for manual volume.
2) For my surprise they both have a Transformer at the output, (as a SET power amp), but the Frequency Band are extended and go to 200Khz.
3) They are made in 120V(USA) and 230Volts(Euro Zone), no 220Volts for my local tension mains.

As Iam new to Valves, I have never see a Output Transformer Coupled preamp before. So, having some doubts, I wish ask to the friends Tube Experts in this Forum:
1) Why they put a Output Transformer on these preamps ?
2) There is any benefit in put a Output Transformer in a Preamp ?
3) There is any problem in a motorized Volume Control device, as many tube preamps makers do not use it?
Thankyou for your opinion, Gustavo

[JoshK]

I'll give you my frank opinions, take them for what they are worth.

Tranformer outputs can apparently be a blessing, but there are always trade offs like with anything.  Usually transformer outputs are done in pretty expensive amps as transformers are much more expensive than caps.  See, the transformer is typically used in lieu of cap coupling the output. 

Some find that transformers sound much better than caps.  However, the rub is that they can sometimes limit the frequency bandwidth.  They claim they go to 200Khz.  That is possible, but typically at the expensive of bass.  The other way involves parafeed arrangement, but that introduces the cap back into the equation.

I am guessing here cause I don't know the topology of the preamp.  I see what looks like 4 6SN7s.  So this suggests they are not doing a simple one triode per channel grounded cathode loaded with the transformer output.  6SN7s wouldn't be the best tube for that arrangement as the rp is too high and there really isn't a good way to extend the frequency high up because of it.  So this suggests that they are probably using some sort of cathode follower output into the transformer, but this is a guess.

If you find the schematic, let me know. 

I wouldn't think running 220V on the 230V version would be a problem.  It would lower the operating point less than 5% but the 6SN7 isn't super sensitive to this.  It'll also lower the filament voltage likewise if it isn't regulated DC, but that also shouldn't be a problem, maybe even beneficial.  I just can't see that as being a problem.

The question about motorized volume control is a loaded one.  Its more expensive to do and that is probably the biggest reason. 

Its an attractive preamp by the way.

[FullRangeMan]

I wouldn't think running 220V on the 230V version would be a problem.  It would lower the operating point less than 5% but the 6SN7 isn't super sensitive to this.  It'll also lower the filament voltage likewise if it isn't regulated DC, but that also shouldn't be a problem, maybe even beneficial.  I just can't see that as being a problem.

Dear JoshK,
Thankyou for your input.
I wouldn't think running 220V on the 230V version would be a problem. >> This is a very important info that I already asking to myself.
This info apply to any EURO 230Volts gear that run in a 220Volts line.
TAS magazine issue 189 have tested the integrated Mystere IA-21(pentode), Mr. Dick Olscher said >> ``There is no input voltage gain stage per se , the input signal is directly routed to the first 6SN7, wich is connected as a long tailed-pair phase splitter. The second 6SN7 is used as a driver stage``

Maybe this integrated IA-21 have something like the Preamp... (maybe)
Regards,

[JohnR]

The term I've seen used is WOT - "with output transformer." As well as not needing (with some circuits) a capacitor as pointed out by JoshK, the transformer is used to, in effect, trade off impedance for gain. Line level preamps don't really need a lot of gain, so it's an approach that makes sense - you can drive the transformer off the plate without needing to use cathode followers or SRPP type of circuits. Good transformers do cost $$ though. I've not heard a WOT preamp but have a couple in my long list of "someday" projects.

The use of a phase splitter in your above post implies a push-pull circuit, which seems a bit unusual for a preamp. At any rate, you can judge the sonic results only by listening, but I don't think there's anything inherently wrong with the concept

[ecramer]

Mystere FAQ
Seems they are a upscale amp made by the same people as prima luna

Upscale Audio deals this line

"The preamps use a SRPP (Series Regulated Push-Pull) circuit for high gain, low distortion. An extremely low output impedance is achieved with a variation of the White Cathode Follower. Each preamp has zero negative feedback. They use a single 5AR4 for rectification, followed by a massive Pi network consisting of a big inductor, separate left and right channel capacitors, regulator circuits and DC filament supplies. We continue to use our high quality, heavy duty, 24-step volume attenuator and fast, wide-band polypropylene caps. Like all our products, the pre amps are point-to-point, hand-wired with rigid copper wire, loosely insulated with woven, silk tubing. '
\

[JohnR]

Oh... that's not describing a WOT preamp. Those "transformers" must be power supply chokes... 

[JoshK]

SRPP into a White CF is a bit like an Aikido but not really.  I assumed from the thread title those were Output transformers, but maybe they aren't.   That is the problem with marketing literature translated from Chinese.  A lot is often lost in the translation.

[FullRangeMan]

Dear audio friends,
Specifically in this case, I assure this both Mystere Preamps are OTC at the output, because Durob told me it in email.
Regards,  Gustavo