His Master's Noise: A Thoroughly Modern Tube Phono Preamp

CCS Resistor Values

Hello everyone,

Last evening I finally started working on my build which felt great after so long studying the circuits and sourcing parts.

I started by testing the CCSs on a breadboard, trying to match the MOSFETs and checking the current for the given values in the schematic.

In the first post, SY wrote:

R5, R14, and R18 set the CCS currents, 20mA for the first stage, 10mA each for the next stage and the buffer. The values may have to be adjusted slightly, depending on the particulars of your batch of MOSFETs; this is most easily done by attaching the + end of the CCS to a power supply (24V or more) and the – end to a 100R dummy load resistor connected to ground. The 20mA CCS should be adjusted to get 2V across the dummy load, and (naturally) the 10mA CCS adjusted to get 1V. Exactitude isn't critical.

And now I have a few questions:

With R5 = 120R I could only get around 12mA on the CCS, which is quite far from the 20mA. I had lower this value to 82R, not a slight adjustment, and was able to find two pairs of MOSFETs that would give 19.35mA and 19.65mA with this value.

Does this sound about right? My workbench power supply only does as high as 23.5V, could this be a problem and cause the tests of the CCS to deliver much less current than expected?

For the 10mA CCSs I had to change R14 ad R18 from 300R to 200R, 220R and 240R. The MOSFETs I got are far from matched but using values in this range I was able to get CCSs with a current ranging from 9.5 to 9.9mA.

Will this be a problem? Having CCSs with different resistor values? Or the only thing than matters is that the current is right. I guess different resistor values are fine as long as the current meets the target spec. I wasn't expecting to have to change the resistor values this much. This the 300R value I was only getting around 6.5mA for these CCSs.

Other than that everything went well. I used masking tape to stick the MOSFETs pairs on a sheet of paper and labeled everything so I can solder then once I clear these questions.

Cheers,
Luis
 
Hello everyone,

With R5 = 120R I could only get around 12mA on the CCS, which is quite far from the 20mA. I had lower this value to 82R, not a slight adjustment, and was able to find two pairs of MOSFETs that would give 19.35mA and 19.65mA with this value.

Does this sound about right? My workbench power supply only does as high as 23.5V, could this be a problem and cause the tests of the CCS to deliver much less current than expected?

For the 10mA CCSs I had to change R14 ad R18 from 300R to 200R, 220R and 240R. The MOSFETs I got are far from matched but using values in this range I was able to get CCSs with a current ranging from 9.5 to 9.9mA.

Will this be a problem? Having CCSs with different resistor values? Or the only thing than matters is that the current is right. I guess different resistor values are fine as long as the current meets the target spec. I wasn't expecting to have to change the resistor values this much. This the 300R value I was only getting around 6.5mA for these CCSs.

Other than that everything went well. I used masking tape to stick the MOSFETs pairs on a sheet of paper and labeled everything so I can solder then once I clear these questions.

Cheers,
Luis

The mosfets are quite variable. Dont worry if the resistor values are quite different.

I used a trim pot set up as a variable resistor to find the value of resistor i needed to set the current. Once set, take trim pot out and measure with multimeter. It really doesnt matter if you use combinations of series/parallel to get the value you require although I would start with the first resistor value higher than the trim measured eg, if the trim measures 110r, then use a 120r, and use higher values of resistors in parallel to knock it down. In the case of 110r you may find 2 220r in parallel is a better bet. Again, try several different resistors of the same value to get the perfect match.

THere are 2 mosfets in the circuit. 1 position is sensitive to the variance of the mosfet, the other isnt. I cant quite remember which though. If you are OCD about matching the mosfets, figure which mosfet changes the current and then you can swap it out to find another that matches. (probably turning off the power supply first).

23.5v is fine for testing, they start doing their thing around 14v or so. I think the test circuit in the article suggested 2x 9v batteries. As you monitor the voltage across the test resistor youll see that it suddenly starts working correctly and stabilizes.

The current matching is the issue here. Absolute value isnt too critical. There was a long discussion about this earlier in the thread and a lot of people are running the CCS considerably lower than whats suggested in the article.

Hope this helps.

D.
 
Hi luis.martins,
are you got the right resistor for a correct css?
i used sometimes a CSS of 12mA in the fist stage. i dont remember the worth of the resistors. I´m now using 68 Ohm for one channel and 82 Ohm for the other channel.
This gives me 19,65 and 19,72 mA. The sound is the same.

Hi craigtone,
this is not a MM amplifier. it was designed for MC.

best regards Frank
 
It doesn't have particularly good bandwidth and as its designed for line level I suspect it wouldn't be great at something super low level like a mm cart. I hear what you're saying about the gain though using 1:1 rather than 1:10 but there were several posts on this thread exploring using this as a mm and general consensus was there was a bunch of things wrong.
Isn't there a mm that sy designed? There's also the pass diy pearl , I think one of those is suitable for mm.

Have a look at the equal opportunity phono stage. Out of interest which cart is it for?
 

6L6

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The Soundsmith cartridges are Moving Iron and approx. 2mV output. It's really made to have a 47K resistive load. It would be really interesting to discover a transformer that would work in this situation.

I'd send an email to Soundsmith or pick up the phone and see what they suggest.
 
The cartridge is new. I have had MC in the past but with the step-ups required, I went to Moving Iron instead and have been very happy. They have the detail of MC without the cost of extra gain. I currently have a Pass Pearl 2. It is a great stage but I am looking to build a tube stage. I have a highly-modified EAR clone and my own design on the breadboard now that I have been experimenting with but noise is an issue as well as tube microphonics. That is what brought me to this thread.

I dug through my Linear Audio magazines and found OP's article on his dedicated MM stage so I think I will give that a shot first.
 
The EO that mrdave45 mentioned is the Equal Opportunity phono pre-amp designed by SY as a MM alternative to the His Master's Noise. It makes design choices that are better suited to MM, but it comes fro the same design concept. I have the EO and use it with a moving iron cart, Music Maker III, and it works great. If that's the one you are considering, I highly recommend it. You may even find that jacinnj has pcbs available.

And of course, there is a thread.

"Equal Opportunity" MM Pre

Jac