Looking for recommendation for Phono Stage

What people are working on at the moment
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#1 Looking for recommendation for Phono Stage

Post by RhythMick »

Now I've finished my lovely power amp (well, bar a bit of a buzz to get rid of) I'm starting to look for circuit suggestions for the phono stage.

The cartridge is a very early Koetsu Rosewood MC producing about 0.4mV (is that RMS ?). Recommended loading for the cartridge is I believe 30 ohms, though there are all sorts of specs out there. I had it rebuilt by Koetsu who were pleased to see an original - the specs which came back said 30 ohms so I'll start there and can experiment.

A friend built a very nice sounding balanced phono stage using Hashimoto transformers (1:40 step-up) - I have a pair of those and also a pair of Tribute wound by Pieter as 1:40. 48k total load reflected back through the 1:40 should give me 30 ohms load - let me know if I've missed something as I'm not an expert on this. I could simply copy his design, which used 12AU7 as input and driver valves. I suspect there are better valves for this purpose though.

I plan on using the cartridge outputs floating across the step-up transformer. My power amp is balanced input only and I plan on keeping the signal balanced from cartridge to speaker (well, to output transformer primary). 0.4mV through the 1:40 SUT will give 16mV, but if that's split across 2 phases the grids of the first valves will see 8mV. I'm looking to get somewhere around 2V per phase at the balanced output. Volume control is a balanced autoformer (Tribute).

I recall reading an article about the Morrison Siren Song which was interesting - I've found the PDF for that and will go through it again. Any other classic balanced designs recommended ?

I'd like to consider using DHTs, but I don't think any will give the gain I'm going to need. Might be interesting to look at DHPs? Any suggestions for directions to research ?

Lastly, I'm not averse to using solid state to help maximise the clean gain. I've little experience with it, but happy to experiment.
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#2 Re: Looking for recommendation for Phono Stage

Post by IslandPink »

This is a very interesting subject which I will think about and come back to. You are in good company here with Nick and Stephen.
A DHP would be cool, look no further than the 4P1L which I use as a driver for 300B etc... however microphonics are always an issue here.
There's a lot on old & recent threads about 2-stage amps using D3a or similar, here.
"Once you find out ... the Circumstances ; then you can go out"
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#3 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Thanks yes I've seen several threads on DIY audio over the last few years about the 4p1l, this is what I had in mind. Haven't seen anyone try and use them in balanced circuits, wondering if matching would be tricky. Isolated and vibration damped sockets seem to be important. Plentiful and cheap, which makes a nice change.
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#4 Re: Looking for recommendation for Phono Stage

Post by IslandPink »

I wouldn't want to try them in anything other than the final gain stage.
I suppose one circuit that might be of interest is Allen Wright's RTP3C/D which uses cascodes ( ECC88 ) which ought to be replaceable by a pentode.
Will be on Stephen's ( Izzy Wizzy ) site.
http://www.izzy-wizzy.com/audio/preampnew.html
Can't find the database of circuits just yet.
"Once you find out ... the Circumstances ; then you can go out"
User avatar
izzy wizzy
Old Hand
Posts: 1498
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#5 Re: Looking for recommendation for Phono Stage

Post by izzy wizzy »

Here's a gazillion ideas
http://www.izzy-wizzy.com/audio/phono/

If you find any out and about, tell me and I'll add them but only if I like them ;)

Cheers,
Stephen
User avatar
izzy wizzy
Old Hand
Posts: 1498
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#6 Re: Looking for recommendation for Phono Stage

Post by izzy wizzy »

IslandPink wrote: Thu Jan 17, 2019 8:13 am I wouldn't want to try them in anything other than the final gain stage.
I suppose one circuit that might be of interest is Allen Wright's RTP3C/D which uses cascodes ( ECC88 ) which ought to be replaceable by a pentode.
Will be on Stephen's ( Izzy Wizzy ) site.
http://www.izzy-wizzy.com/audio/preampnew.html
Can't find the database of circuits just yet.
No one has built the one I have but variations of previous ones are out there and Mark's is along the same lines. In fact so many people end up with something along those lines, there must be something in it. Hi gm 1st stage, 20k-ish passive EQ, something to follow depending on gain (usually highish gm), some kind of output coupling. PSU all passive or some kind of shunt. Heaters passive or some kind of current source/shunt reg.

Cheers,
Stephen
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#7 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Some very interesting designs there Steve, thanks. Glad I'm going skiing for a week so I can browse !

Too much choice though. Let's break it down into first principles.

Balanced input is a given (floating from cartridge pins across step-up transformer 1:40).

Balanced output to power amp via balanced 4-way autoformer, ideally at similar level to my DAC/Streamer (Cambridge Audio Azur 851N, love this component by the way). The autoformer is rated to 7v. The first stage of the power amp is 26 with 9v bias, so there's plenty of headroom.

So would this be better ...

1) kept balanced through the gain and EQ stages
2) balanced gain stages with SE EQ (perhaps transformer coupling)
3) SE with balanced output

Gut feel says 1 or 2.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#8 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Also - valve considerations should really start with what I have plenty of stock of ...

26 - plenty of ST, some Globe
71A - short on the Globe ones which I prefer
45 - plenty of both globe and ST

The above are used in the power amp. I also have the following which are interesting and would like to experiment with.

10Y / VT52 - These look really interesting, I may try these in place of the 71A at some point.
01A - Half the current drive of the 26. I will try these in the power amp at some point.
6SL7/5691 - lots
6SN7/5692 - lots
6DJ8/6922 family - lots including old PQ variants and also 7DJ8

Finally, I just ordered some 4P1L as I know I'm going to want to play with them.
User avatar
izzy wizzy
Old Hand
Posts: 1498
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#9 Re: Looking for recommendation for Phono Stage

Post by izzy wizzy »

RhythMick wrote: Thu Jan 17, 2019 10:34 am So would this be better ...

1) kept balanced through the gain and EQ stages
2) balanced gain stages with SE EQ (perhaps transformer coupling)
3) SE with balanced output

Gut feel says 1 or 2.
Mine is option 3 :)

cheers,
Stephen
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#10 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Yeah I saw.
User avatar
izzy wizzy
Old Hand
Posts: 1498
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#11 Re: Looking for recommendation for Phono Stage

Post by izzy wizzy »

RhythMick wrote: Thu Jan 17, 2019 11:25 am Also - valve considerations should really start with what I have plenty of stock of ...

26 - plenty of ST, some Globe
71A - short on the Globe ones which I prefer
45 - plenty of both globe and ST

The above are used in the power amp. I also have the following which are interesting and would like to experiment with.

10Y / VT52 - These look really interesting, I may try these in place of the 71A at some point.
01A - Half the current drive of the 26. I will try these in the power amp at some point.
6SL7/5691 - lots
6SN7/5692 - lots
6DJ8/6922 family - lots including old PQ variants and also 7DJ8

Finally, I just ordered some 4P1L as I know I'm going to want to play with them.
If you're going to try and use DHTs in a phono, good luck. That will be very hard but not impossible coz nothing is impossible. Thomas Mayer did one and there's one of his and some others in that collection linked before.

Here's my take on mucking around in this area and taking a great interest in what others have done. All IMHO of course. You need big gain in the first stage. I used to pooh pooh the hi gm approach coz gain is gain right? I fought the hi gm route for years. Then just for giggles I tried it. I wouldn't go back. It's where most people end up not all and there's a reason. Hi gm valves are a PITA and some don't want the hassle.

The 2nd thing hi gm valves enable is a lowish Z EQ - say a series resistor in a passive circa 20k. I think this is a game changer from EQs with a series resistor circa >100k.

And then when some have gone to here they take the next step, usually a final one to LCR. I haven't due to cost otherwise I would.

2nd stage is for flavour, gain and output configuration.

Thoughts for starters.

Cheers,
Stephen
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#12 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Thanks Steve.

1. First stage high gain GM ... Agree. Don't think DHT are going to cut it there. DHP perhaps.

2. EQ ... Sorry to ask the potentially dumb question, but what's LCR?
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#13 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Oh I must be tired. Choke, cap, resistor. Sigh. I'll get me coat.
User avatar
izzy wizzy
Old Hand
Posts: 1498
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#14 Re: Looking for recommendation for Phono Stage

Post by izzy wizzy »

RhythMick wrote: Thu Jan 17, 2019 8:10 pm Oh I must be tired. Choke, cap, resistor. Sigh. I'll get me coat.
Or too excited about skiing. Where are you going? I'm off to 3 valleys beginning Feb. Can't wait.

cheers,
Stephen
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#15 Re: Looking for recommendation for Phono Stage

Post by RhythMick »

Serre Chevalier. Favourite resort, can't wait. Fresh snow today I hear
Post Reply