SS CFP RIAA preamp

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JamesD
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#1 SS CFP RIAA preamp

Post by JamesD »

Here is the present circuit for the SS phono amp I'm knocking up. Its based on two CFP gain blocks and a passive CR RIAA filter between them and powered by two PP9 batteries in series - designed around 15volts but happy from 18V down to 12 Volts - its ac coupled in and out
CFP RIAA CR filter.png
It has a LR counterpart that I will post tomorrow based on the Pete Millett LR phono stage only with the CFP gain block rather than op-amps and using the DonAudio inductors rather than the Cinemag inductors Pete used (although the Cinemags are available from DonAudio if you want to use them...)

After convincing myself that the LR filters sound better I expect to build a valve based phono as the final version so these are steps on the way to that goal...
JamesD
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#2 Re: SS CFP RIAA preamp

Post by JamesD »

Here is the LR version using DonAudio inductors and inspired by Pete Millets LR phono preamp.
CFP RIAA LR filter.png
Pete Millets article is here http://www.pmillett.com/LR_phono.html

and the DonAudio inductor is here https://www.don-audio.com/index.php?a=186&moreRating=1

Again battery powered - just for ease of use as these are temporary designs for me to decide which is best to proceed with as a find either tube or tube ss hybrid design...

ciao

James
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IslandPink
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#3 Re: SS CFP RIAA preamp

Post by IslandPink »

Those chokes are not too expensive are they ?
Hopefully you'll get lots of Leistung for the Preis .
I must sit down later & compare that equaliser to what Steve Bench did - it looks different.
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JamesD
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#4 Re: SS CFP RIAA preamp

Post by JamesD »

They are good value and seem to be consistent - although their spec is only +/-10% so absolute value would need checking and the Rs tweaking to maintain the riaa spec to say 1dB but its possible to buy them in matched pairs where they are matched to better than 1% for a small extra cost so channel balance can be maintained. the reviews of the don audio inductors state they are not quite as transparent sounding as the very best options but they have a 'sweeter' sound than those... so they should be good enough to evaluate the CR against the LR...

Its quite different from the Steffie's design and from the Tango LCR beastie - its worth reading Pete Millets writing on what he did and why...
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#5 Re: SS CFP RIAA preamp

Post by JamesD »

I knocked up a birds nest version of this and once I stopped it oscillating :D it was good but not exceptional. What it did was knock my step up transformer out of the park and allowed my mc cartridges to clearly beat the mm cartridges...

But it is a pain to keep adjusting the gains between the 70uv output of the Ortophon SL20E and the 300uV of the Denon 103!

I noticed that the current generated by the two catrtdges is similar so I'll be designing a tweak version of the Richard Lee " Duraglit" current input head amp - when I get my head back on properly...

I've just finished five hours of Chemotherapy today so that will knock me out for three days but more after that...

ciao James

p.s. An example of how my head is all over the place.... this should have been posted in this thread "SS CFP MC Head Amp" rather than the RIAA version of the thread.... Sorry!!
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Nick
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#6 Re: SS CFP RIAA preamp

Post by Nick »

JamesD wrote: Sat Nov 14, 2020 10:53 am Here is the LR version using DonAudio inductors and inspired by Pete Millets LR phono preamp.

CFP RIAA LR filter.png

Pete Millets article is here http://www.pmillett.com/LR_phono.html

and the DonAudio inductor is here https://www.don-audio.com/index.php?a=186&moreRating=1

Again battery powered - just for ease of use as these are temporary designs for me to decide which is best to proceed with as a find either tube or tube ss hybrid design...

ciao

James
Looking at this, the cynic in me can't help smile at a LR EQ stage with a 1mF cap on either side of it :-). I wonder if you could get the bias to work directly coupled with a single cap below the EQ network to ground
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JamesD
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#7 Re: SS CFP RIAA preamp

Post by JamesD »

Almost certainly - not done the LR version yet - still with CR as a trial and then get the LR version going... its the dc current through the inductors that dictated the conservative approach to start with... although that suggest a cap into the network and dc connection out maybe... need to think this through...

Thanks for the observation, Nick...
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Mike H
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#8 Re: SS CFP RIAA preamp

Post by Mike H »

Can someone define 'CFP' please
 
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JamesD
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#9 Re: SS CFP RIAA preamp

Post by JamesD »

Complementary feedback pair

Its a nice pair where the first transistor defines the pair gross characteristics and the second transistor provides the current umph and both contribute to the gain. The potential problem with them is that they are prone to burst oscillations as there are multiple feedback paths that can negate the usual collector to base capacitors effectiveness... The big gain for power stages is that the first transistor defines and controls the thermal runaway properties so mount the first transistor away from the heat-sink...

The limit for the power case is the Sziklai pair used for quasi complementary output stages.

Hope that helps.

James
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Mike H
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#10 Re: SS CFP RIAA preamp

Post by Mike H »

Yes i does, thanks! :thumbleft:
 
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