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6SL7 Mu Follower for CD Player

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Posted on February 3, 2023 at 12:47:03
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 I am in the process of 'lampizating' a Marantz CD player with TDA1541A in NOS mode.
I plan to use a 6SL7 mu follower for the output stage.
I have looked at many examples of output stages, all seem very good and I'm now a bit confused to be honest!
Lampizator typically uses SRPP to good effect but I guess a properly configured mu follower could be better. I have several 6SL7's, hence my choice.

I plan approximately:
B+ 380V
6SL7 at about 160V anode, -1V bias, 1.9mA, both valves.
30kohm load resistor on the lower anode.
Take output from upper valve cathode for reasonably low Zout.

I wonder if -1V bias will be OK; it looks OK on the curves, and the input signal swing will be tiny so I expect not to suffer grid current (I can't find data on this for the 6SL7).

I could go for marginally less bias to give more current, say 2.2mA at 0.9V bias approx; any comment?

My gut feel is that a 6SL7 mu follower like this could give the gain I'd like with a very good sound. Worth trying?

 

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6SL7 w/ MOSFET source follower, posted on February 8, 2023 at 07:14:55
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 Simpler, and "better": Just use ONE 6SL7 at 150v 1.7ma, one triode for each channel.

380v B+. (230v drop to get to 150v). 230/R=0.0017. Plate R=135,300 ohms

Cathode at around 1.25v @ 1.7ma. 1.25/R=0.0017. Cathode R= 735 ohms
You might have to vary a bit from +/- 735 ohms to get your tube in at 1.7mA

Bias the source follower with 50,000 ohms source to ground.

 

RE: 6SL7 w/ MOSFET source follower, posted on February 12, 2023 at 06:18:40
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 Good suggestion; in my research I've found similar and I don't rule it out, I may try it among other things later.
I have used a pentode active load in Kimmel mu stage cct many times with good results - with a 27 and E83F it makes a great line preamp. I note the mosfet is somewhat similar to a pentode.

My priorities are vocals and tonality, so (though I may be wrong!) I tend to avoid solid state. I'm building the 6SL7 mu follower as I think it'll be interesting, simpler than mu stage and only 2 valves needed.

In future I may try a simple triode cct. But good sounding triodes with high gain and low Ri seem a bit hard to find; but choke loading might work very well.
And of course a simple 6SL7 followed by a CF ... perhaps a CF with CCS load. Or I might think of a CF with a high negative PSU to the load resistor, giving a high value load and so somewhat like constant current.

 

RE: 6SL7 Mu Follower for CD Player, posted on February 14, 2023 at 20:51:16
DHT 4 ME
Audiophile

Posts: 564
Location: Las Vegas
Joined: November 9, 2005
 While the mu follower is an improvement over the SRPP I have never heard a high mu tube in mu follower work as well as the asymmetrical mu-follower. There was a great article in glass audio that is in the archives and it gave several configurations with calculated values and calculations for the circuit.

If you are not familiar the circuit uses a low mu, low Rp top tube and the gain structure is set by the lower tube choice. This gives optimal tube types in their respective positions. The first circuit I did was a doubled 6sn7 w a triode connected el84. It was s sonic breakthrough for me.
Robert
www.Robert-Park.com

 

RE: 6SL7 Mu Follower for CD Player, posted on February 19, 2023 at 01:23:06
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 Thanks for this! Noted, and will be tried.

As I use one valve for both lower triodes and another for both uppers (to allow heaters to be floated a bit above cathodes), I can easily try a 6SN7 upper valve with just a change of its Rk.
Current could probably br raised to about 2.5mA (I have not yet looked into this), a bit low for 6SN7 I guess but worth trying.

The unit is almost built so I'll try it as is, mu follower, but I'm sure I will be trying other circuits in future. I'm wondering about a kimmel mu stage cct but with an upper triode not pentode, as space is very limited, I'd like to stay with 2 valves.
I'm also tempted by a simple 6SL7 1st stage with anode R followed by a CF with a high negative voltage on the load resistor. I may try this in time.

It's always hard to find the 'best' way to do something but I think everything in this thread should be pretty good, worth trying.

 

RE: 6SL7 Mu Follower for CD Player - Working, posted on February 23, 2023 at 11:35:52
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 It sounds good to me, no doubt about that. Musical, rhythmic, foot tapping sound, better than expected in this respect.
Tonality is good, fine female vocals.
I'd say the sound is detailed, clean, neutral as expected from a linear valve in a low distortion circuit, and that's fine by me. The dac chip is good, doesn't need massaging by an over sweet amp stage.

Everything seems OK with the sound of this lampizated CD player. I can't think of anything wrong, it is not slow, rolled off or over sweet; it does have valve sweetness and sound quality generally.
It is a modest Marantz CD583 with TDA1541A, upgraded with good caps in the PSU, NOS converted, oversampling chip removed, improved clock. Now with the valve output, I'd say it is seriously good (due mainly to the excellent TDA1541A); and I have some good digital gear such as Cayin CD100i upgraded, ANK DAC 4.1x, Aqua La Scala (latest version), and it is in the same class. When run in I expect it to sound usefully better than the Cayin, and in some respects I may prefer it to the dacs.
In other words I put it in the £2000 class and just might reach £5000 ... what matters most is that I like it a lot, and expect to love it.
Many other folks report similar results from lampizated TDA1541 players!

The mu follower is as follows.
PSU is CLC then LC left, LC right. 1st choke 20 H, stage 2 are 8.5 H miniatures 950 ohm. Caps are 15uF and 20uF motor run, then 40uF DC-links in the final stage.
B+ 350V, a bit less than expected.
I/V conversion resistor is 51 ohm AMRG; in effect it is the grid resistor.
Anode load 33kohm, AMRG
Bias resistors 510 ohm takman carbon, upper grid resistor 100k takman carbon.
Coupling caps 0.1uF Vcap ODAM - my favourite coupler, highly natural.
Output caps Mundorf Premium SGO 1.5uF.
Runs at about 1.7mA, a shade lower than planned - but may come up with valve rolling.

If nothing else, this cct will prove a good benchmark to evaluate other options which I intend to try, probably with an asymmetric mu follower first which could well be an improvement.

 

RE: 6SL7 Mu Follower for CD Player, posted on February 23, 2023 at 12:00:06
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 Any suggestion for the upper valve of an asymmetric mu follower?

With care I think I can run a 6SL7 lower valve at say 2.7mA; grid bias will be low but I think that's OK as the input signal is tiny.
So I need something that runs well at this low current; I guess it may be too low for a 6SN7 to work well, but you experts may know better.

I've looked into some regulars like 12AV7, E180CC with usefully higher Gm; any comment? I like the fairly high gain in a CF.

While writing this I thought of E80CC which I know sounds good; curves look reasonable at 2.5mA, and Gm about 2.5mA/V (at low current, guess) Zout should should be about 400 ohms, definitely low enough. I'm tempted.

Any advice welcomed, TIA.

 

RE: 6SL7 Mu Follower for CD Player, posted on February 23, 2023 at 12:14:02
DHT 4 ME
Audiophile

Posts: 564
Location: Las Vegas
Joined: November 9, 2005
 The top tube can run at higher current, it just drops more voltage.
I really love the E80CC tube for gain stages and it probably will work for a top tube well.
Something like the EL802 is a killer tube that even surpasses the D3a at a very affordable price. You would have to dig up the article and try to map out the op points and your current PS voltage may dictate what you can get away with.

Here is a broad set of tests that Pete Millet did a while back on high GM pentodes run as both pentode and triode.

http://www.pmillett.com/pentodes.htm


PL802 tested is the 16v heater version of EL802. Really amazing results in some of these tubes at great undiscovered prices.

Robert
www.Robert-Park.com

 

RE: 6SL7 Mu Follower for CD Player, posted on February 26, 2023 at 06:58:52
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 Thanks Robert for the good info and taking the trouble to help.
I have some relevant experience as follows.

I had a Basie preamp (diyhifisupply), ECC82 mu follower and CF. With a friend, we raised the B+, fitted E80CCs and removed the CF, taking output from upper K of mu follower. Current was raised a bit but IIRC little over 2mA. Results were very good and it stayed in his very high end system for a long time.
Having heard the E80CC run as a ECC82, I tried it at a better op point, about 5mA. I also used the Kimmel mu stage cct with E83F pentode on top, in Nina preamp.
Results were very good indeed, significantly better than the modded Basie. Later I made my present 27/E83F mu stage pre, even better, exceptional.
I think the E83F is exceptional. Runs about 10mA Ia so fits very well in a mu stage with 5mA in the triode. It is used in a microphone amp so it is very quiet, and I find it to sound good; I have compared to E180F Mullard and D3a in this function, it was better.

Anyway my experience with Basie suggests that E80CC works well as a mu stage upper valve; Nina is better, but much of that may be due to running at a better op point and having a better PSU, how much is due to the E83F I do not know.

So I'm thinking what to try next.
6SL7/E80CC mu follower; only 2 valves is good as space is tight, and I know the E80CC can work OK at a bit over 2mA.

6SL7/E80CC in a triode version of Kimmel mu stage, 2.6mA in triode, 6mA total in E80CC. Could be very good indeed, great sounding valves at very good op points; but somewhat unknown. Worth trying I guess.

6SL7/E83F Kimmel mu stage, 2.6mA in the triode, 6mA in E83F which looks to be a good op point; but a struggle to fit 3 valves in. But might be the best sound of all.

But just to add to the confusion, the 6SL7 mu follower is running in and sounding very good indeed which suggests staying with this cct, putting E80CC on top. I guess I'll have to try everything in time. No way will I change the existing setup, it is too good to mess with.

Just a note for possible lampizators; the results with the cheap Marantz are very good indeed; this valve stage gives clearly audible improvements, it is not subtle! I think the 6SL7 gives good very low level detail so resolution has improved a lot, as well as tonality and sweetness. Though I'm looking to go further, anyone building this 6SL7 mu follower is unlikely to be disappointed in any way!

 

RE: 6SL7 Mu Follower for CD Player -Result, posted on March 2, 2023 at 03:37:03
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 Now it has run in, I find the resulting sound is excellent.
This is a medium quality Marantz player (CD583), TDA1541A chip. It was reworked by ebay seller amp-king2 in the UK, with upgraded caps, NOS conversion and improved clock.
Results with the valve output stage are amazing. It substantially outperforms my modded and reclocked Cayin CD100i in very musical and enjoyable ways; more detail, transparency, body to vocals, tonality, rhythm. It also soinds better than an Aqua La Scala dac with top Pro-ject transport, in terms of tonality, transparency, naturalness, just sheer sound quality.
Bloody lovely sound!

B+ came out a bit lower than expected so the current is 1.7mA, not 1.9mA as planned; doesn't seem to matter, but I may try a different mains TX some time.
If anyone is thinking of lampizating a CD player I can recommend this 6SL7 mu follower; details are in another post of mine below.

 

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