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-   -   RIAA triode or pentode (http://www.diyaudio.com/forums/tubes-valves/225777-riaa-triode-pentode.html)

Gluca 15th December 2012 08:57 PM

RIAA triode or pentode
 
well I know ... there are zillions of discussions and schematics out there and I am considering many of them, but would somebody of you share bad/good experience with pentode RIAAs?

gianluca

kevinkr 15th December 2012 09:23 PM

Use a high transconductance low noise type like the D3A(7721) or E810F(7788) and it should work quite well based on my very, very limited experience. Input capacitance will be quite low with just a little care.

Another option is a cascode built around the ubquitous ECC88, Russian 6S3P or 6S4P, or the 5842/417A - any of these will also give you low miller capacitance as the lower tube in the cascode will have relatively low effective mu.

Gains of 46 - 50dB or more are achievable in the first stage with care.

Gluca 15th December 2012 09:27 PM

Kevin, do you mean triode strapped D3As or E810FFs or just plain pentodes?

G

kevinkr 15th December 2012 09:34 PM

Hi Gluca,
No, I actually mean pentode connected D3A, and 7788.. Or alternately triode cascodes which could incidentally include a D3A in triode connection.

I made the mistake of using the D3A in triode in the design prior to my current muscovite design (analog source) and the total miller capacitance and strays at more than 300pF is a killer.. This is not the best sounding phono stage I have designed by a long shot, how it interacts with the cartridge is a big problem eliminating most MM from consideration.. :D

Merlinb 15th December 2012 11:03 PM

I guess the reason you're asking is noise. With triodes, to minimise noise you need to operate at the lowest possible anode current you are happy with. High gm helps too, but anode current is the main thing to think about.
With pentodes there is additional noise introduced due to screen current partition. To minimise that you need to minimise screen current relative to anode current. In other words, you need to run with low anode current, very low screen current, and high gm is a bonus if you can get it.

Having said that, pentodes are probably not as bad as some would have you believe. For example, the equivalent input noise of an EF86 over the audio band is only about 2uV, whereas a reasonable triode might be 1.5uV. (OK, a really well chosen triode might achieve 0.5uV, but be prepared to pay for Telefunkens or something!)

You could of course use a triode at the input, but pentodes for later stages where the noise is no longer important, but then you may have to think about linearity instead.

Chris Hornbeck 16th December 2012 12:11 AM

There's a must-read article in the first (Volume 0) issue of Linear Audio. Frank Blohbaum (mit der Umlaut) has an eye opener about pentodes and noise.

All good fortune,
Chris

Eli Duttman 16th December 2012 01:15 AM

TANSTAAFL will be with us forever. Name your poison: pentode partition noise or triode Miller capacitance. Cascodes are interesting because CMiller is low, along with avoiding partition noise. The "gotcha" with a cascode is abysmal PSRR. IMO, hybrid cascodes of either permutation deserve consideration. Allen Wright successfully stacked a high gm triode on top of SS, but a high voltage MOSFET stacked over a high gm triode bears looking into too.

A high μ, high gm, low RP, low CMiller triode to keep in mind when scoping a RIAA preamp out is the 6GK5. A low noise pentode to consider is the Octal 6AC7. Don't forget the silicone rubber damping rings and DC heater supply.

Chris Hornbeck 16th December 2012 02:18 AM

Frank Blöhbaum

Chris

revintage 16th December 2012 08:46 AM

Note Frank Bs circuit is made for high NFB applications. His PFL200 phonostage has an open loop bandwith of 13kHz and ca 35dB NFB. Unfortunately he has removed his schematics from Intactaudio/forum.

SY 16th December 2012 09:47 AM

Quote:

Originally Posted by Chris Hornbeck (Post 3286696)
There's a must-read article in the first (Volume 0) issue of Linear Audio. Frank Blohbaum (mit der Umlaut) has an eye opener about pentodes and noise.

Yes, I saw his presentation at ETF and read the article. What's interesting (to me) is that he hasn't really overcome the partition noise issues in pentodes. His circuit, at essence, is a cascode with a bipolar on top, triode on bottom with the screen used as a plate. The plate in his pentode has hardly any effect on the circuit- I built one on the bench, disconnected the plate, and got almost identical results.

This isn't that surprising since using the screen of a pentode as a plate in low noise circuits is a long-known trick.


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