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Hybrid RIAA tube preamp SS Poweramp General Arrangement


Could someone please confirm what the big picture general arrangement to drive a pair of SS Icepower Monoblocks with a tube preamp from a turntable phono signal? Looking to only use tubes and no solid state up until the poweramp input. Assumption; SS Amp has 10K impedance so preamp requies <1K impedance.

Example 1. - Preffered but cannot find design
Phono MM or MC cartrige > Class A Single End Tube Preamp (impedance <1K) > SS Monoblocks (10K impedance)
Can anyone recommed any Class A Single End preamp designs with an impedance of less than 1k that accepts a RIAA input?

Example 2.
Phono MM or MC cartrige > RIAA Tube Preamp > SS Monoblocks
such as http://diyaudioprojects.com/Tubes/RIAA-Phono-Preamp/

Example 3.
Phono MM or MC cartrige > Tube RIA converter > Class A Single End Tube Preamp > SS Monoblocks
I found this design but I think it does not accept a RIAA phono input
My take, for whatever it's worth...

A solid-state or Class D amp is going to have input impedance of 10k or possibly up to 50k ohms. That's too low for the old/classic "RCA" 12AX7-based tube phono preamp. Most people who use something like that add a buffer on the phono preamp's output to make it able to drive cables and solid-state amps' low input impedance. Either a tube cathode follower or a MOSFET source follower will do the trick nicely. Easy peezie. Many have designed and built tube phono preamps like this with output impedance of <1k ohms.

So, one solution could be: Turntable > Tube phono preamp (including the RIAA filter) with output buffer > Passive input switchbox/10k ohm volume control > Power amp.

The above example assumes a MM cartridge. For LOMC, you'll want to add some kind of step-up transformer or solid-state 'MC head-amp' (JFET, opamp, etc.).

However, if you also have digital sources (CD player, DAC, etc.) then you might want to put a tube in the signal chain between that and the power amp. That's where problems happen.

Problem 1: Too much gain -
  • Today's Class D amps have about 18X (25dB) gain, so they can be driven to full power straight from the outputs of any digital player, even from a smartphone. No additional gain (as from a preamp) is necessary, or even wanted.
  • Most triodes run open loop will have gain of anywhere from about 5X (14dB) to 20X (26dB). That's too much gain.
Problem 2: The additional gain will inevitably add noise (related to Problem 1).

Problem 3: The additional gain may cause transitory overloading if you listen loud.

Problem 4: You'll be adding harmonic distortion (that might be the goal, though).

You could use a tube cathode follower in a line preamp (control preamp), which will have about 0.9X (-0.9dB) gain. The slight loss of gain will not be noticeable in use. However, a well designed cathode follower operates with nearly 100% negative feedback, so it will play extremely cleanly. Will it 'sound like a tube'? Actually, probably not. Here we have to ask, 'What's the point, then?'

In my current system, I found I like a little tube distortion with my DAC output. OK, I'm silly, stupid, misguided, whatever. That's what sounds more fun to me, so that's what I do. To implement this, I set the maximum output level from my DAC (Allo Boss on a Raspberry Pi) to 1.0V RMS. That's half (-6dB) of the standard 2V RMS max output level for unbalanced digital audio line level sources. I put together a little 'anode follower' to supply 2X (+6dB) gain to bring the max output level from the DAC back up to about 2V RMS. I used a 6DJ8 common cathode stage with about 20dB of local NFB in parallel from its plate (anode) to its grid. It fattens up the sound a little bit. The NFB brings the 6DJ8's output impedance down to about 1k ohms.

Yes, I know I added noise and harmonic distortion to my DAC's output. However, I like the way it sounds (subjectively). The noise is not noticeable when I'm listening at normal volume through speakers, and I guess I like the little bit of added harmonics. That's all.

So that solution looks like this:

INPUT 1 = Turntable > Tube phono preamp with output buffer > Passive input switchbox/volume control > Power amp.

INPUT 2 = DAC output (1V out max) > Tube anode follower stage (2X gain) > Passive input switchbox/volume control > Power amp.

My volume control is an Intact Audio tapped autoformer type, which maintains a high input impedance with low output impedance. A standard 10k ohm potentiometer would work fine for the volume control as long as the signal sources all have low output impedance of 1k ohms or lower.

My point is, it's best to look at the system as a whole, not just a string of individual boxes patched together. As a DIYer, you can be creative in your approach, look at different ways to solve the problem that may not be available from the internet Audiophile Emporium.

Generally speaking:
  • Keep input impedances high.
  • Keep output impedances low.
  • Keep the gain structure so that no source output overloads a load input. Too much gain in the middle of the signal chain can cause overloading on peaks, as well as added noise.

Simple stuff.

BTW, koifarm's hybrid opamp/mu-follower RIAA preamp (post #3) looks very good. The opamp input will have very low input capacitance, so will work well with contemporary MM cartridges and any SUT you choose for a LOMC cartridge. Neat.
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There will be a V2 this summer/fall of the hybrid phono preamp. With standard parts and diy 3d printed housing. Dipswitch, opamp and tube accessible from the top for parts rolling and impedance changes, so you do not have to open the bottom plate of the housing. Gain will also switchable so you can use High MC cartridges.
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Yup. Pretty much every tube phono preamp is going to be single-ended, and therefore Class A by definition. There have been a couple of balanced tube phono preamp designs, but those also run in Class A. Op-amps have Class AB output stages, correct? Maybe that's the distinction being made here. No op-amps allowed?

Here's an all-tube phono preamp with lower than 1k ohm output impedance:

Hagerman Audio Labs makes a kit that's similar (12AX7 > RIAA EQ > 12AX7 > 12AU7 Cathode Follower):

You can read the kit assembly guide and see the schematic there:

Here's another two all-tube phono preamp designs with PCBs available:

All of those use commonly available tubes.

There are many more designs like these. Many, many.
Thank you all very much for your responses, each pose great suggestions and advise. The question sounded vauge and unclear because my knowledge is just that. I am very new to building DIY preamps and genuinly am unsure of the best overall big picture architecture should look like.

I guess I am looking for 2 X DIY designs (stand alone tube phono amp and stand alone single end preamp) that marry up and can overcome challanges that Rongon mentioned about integrating beween a tube pre amp and a SS power amp by using a cathode follower and not using any op amps. Keeping the design deviod of op amps until the off the shelf SS power amp and hopefully one day in the future building a tube power amp to replace it to have a complete tube system.
Still don't understand why you think you need 2. Just find a phono stage design that has a cathode follower output stage. There are many, beyond those that have been identified in this thread already. Just about any cathode follower has output impedance lower than 1K. CF output impedance is approx 1/gm.
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This buffer of my shanling cd-t100se you can also use for a nice passive source select and volume control. Just ad a source select/ volume control board . You have no gain.


Anode supply is 200V/ min 50mA.