JimL

I use Mr. Clean Magic eraser - just shove the pins into it a few times. Also works well for cleaning off cartridge tips for those retrogrades among us who still listen to LPS.

Re: grid stopper for the cascode CCS (BTW, the spell checker seems to substitute cascAde for cascOde unless you manually correct it). The original circuit was taken from Walt Jung and Gary Pimm's work, I used 100 ohms for the DN2540 because that's what Jung used and I figured he knew what he was doing, and 1 kilohm for the 10M90S because that's what KG used.

JoaMat, Very nice adaptation One caveat, the 10M90S is designed as a current source, so I presume that the choosing the current setting resistor can be done using the graph in the data sheet. However the DN2540 is not specifically designed as a current source, so I have found there is more variability in the current from one sample to the next if you're using the same value resistor. This is why in my schematic I use a pot for fine-tuning the current, and also a current sensing resistor (10 or 100 ohms) to set the pot to the desired current. I would also be interested in hearing about any subjective changes you hear after the mod, since in my SRX mod I never tried the 10M90S by itself.

Interesting question, I was wondering that myself. The T2 current sources are pretty sophisticated, but without measuring their dynamic impedance I don't know if using cascoded MOSFETs would give a higher dynamic impedance, which would be the reason for making a substitution. My bias is that the closer the current source/load is to a perfect current source the less it should contribute to the sound.

re: Nikongod: "Why not run the input tail off of B-?" Kevin didn't show my power supply schematic, but that is exactly what I did - calculated the current draw for both input tail current sinks and used a resistor from B-. Re nopants: "isn't using the HV rail like that similar to what the exstata does...?" The current source isolates the input tail from any variations in the power supply, which are minimal since the power supply is regulated. "Any thoughts on using the the output CCS's in combined CCS and mu follower mode?" Interesting idea, I hadn't noticed that before even though I've looked at Gary Pimm's CCS postings several times in the past. Gary specifically mentions that it works well for high capacitance loads, and electrostatic headphones aren't really that high capacitance - around 100 pf give or take. More importantly, though is that I wanted to maintain the "character" of the original SRX circuit. As I mentioned in a previous post, a cascoded 10M90S/DN2540 has a very high measured impedance - I measured a DC impedance greater than 160 megohms, which is my measurement limit with the crude equipment I have. By comparison, a single 10M90S current source measured the same way works out to about 170 kilohms DC impedance. In comparison with all the other impedances in the circuit, the cascoded current sources are orders of magnitude higher impedance. A stat headphone with a typical 100 pf impedance is j80 megohms at 20 Hz, and drops from there at higher frequencies. What this means is that the current sources are almost completely out of the circuit as far as the music signal is concerned, all they do is optimize the function of the tubes as active devices. So effectively all the output tube sees is the headphone, and all the headphone sees is the tube driver. As I said, it converts an amp for driving anode resistors to an amp for driving headphones. By the way, another advantage of having both current sources and sinks on the output stage is that they do a very good job of isolating the output tubes from the power supply and from the other channel. Re: Pretentious fool: My main concern with the DN2540 was whether it would work at currents around 1 mA, since most of the designs I seen using it run much higher currents. Fortunately it seems to be stable at that current, the current didn't vary when I changed the voltage by 10-15 volts, and it's quiet.

A couple more comments: 1) Since the SRX circuit is inherently balanced, it's easy to have a balanced input - just disconnect the grounded half of the input circuit and use it for the negative input. For unbalanced input, just connect the negative input to ground. 2) Haven't tried the LND150 for the input stage current sink, but my unit the DN2540 worked fine. I tried a J113 and 2SK170 as the lower device in the cascode with the DN2540 as the upper device but the DN2540 for both devices worked best. The input current sink runs about 1.3 mA - 650 uA/section - I used the TO92 version rather than the TO220 that Kevin has on the board. 3) Let me re-emphasize that with B+ and B- > 300 volts, you CANNOT use 6SN7 or 5692 tubes. Use 6SN7GTA or GTB tubes ONLY in the output unless you enjoy burning up expensive tubes. The 6SN7 is specified at 300 volts max plate voltage and 5 watts combined plate dissipation, the 6SN7GTA/B are specified at 450 volts max plate voltage, 7.5 watts combined plate dissipation and 1500 peak positive pulse plate voltage. Most of the new old stock (NOS) 6SN7s that are going for silly money are the original version. NOS 6SN7GTA/B are still available at reasonable prices, and there are also new manufacture 6SN7GTAs. 4) I haven't tried this, but if you use 6BX7 tubes for outputs you could crank the power supply up to 400 volts, increase the output current loads to 10 mA and the output current sink to 23 mA - might have to upgrade the heatsinks though. Frequency response should be about the same but more current drive for the headphones.

Thanks for posting this, Kevin. I've submitted a full article to AudioXpress which I hope will be coming out in a few months. It contains a very detailed analysis of the basic SRX circuit and the modifications I have made. In the meanwhile just a few comments. 1) the use of current sources in the output stage is very important. With anode resistors in the output stage, the resistors are much lower impedance than the headphones themselves, whereas with cascoded current sources, the headphones are significantly lower impedance than the current sources. To put it another way, they convert an amp for driving anode resistors into an amp for driving headphones. Because the 6SN7 tubes are not nearly as high power as a EL34, this makes a significant difference. 2) My crude measurements of a cascoded current load puts the impedance at > 160 megohms, which means at 20 Hz about 2/3rds of the signal current is going to the headphones, and by 100 Hz, 90% of the signal current is going to the headphones. 3) all the current sources are adjustable. In the input stage the current source should be set at about 1.3 mA, so that the plate of the upper 12AT7 is about halfway between ground and B+. The output current loads should be set for 7 mA, and the output current sink should be set for about 17 mA. The resistor string between B+ and the output current sink gives a pathway for the excess 3 mA of current difference between the current loads and the current sink. The output current sink should be adjusted to zero the output plate voltage - anything less than 10 volts from ground is acceptable. 4) I'm currently running my unit with B+ at about 325 volts, B- at about -325 volts using a TL431 based shunt regulated power supply. The schematic of that power supply will be in the article. I know there have been experiments posted here using the TL431 for a regulator that commented on turn-on instabilities and oscillation, however I found a Texas Instruments application note on how to correct that problem, and my version seems to be stable. 5) For the input stage I recommend ONLY 12AT7 tubes for the lower cascode tube, as substituting a 12AX7, for example, will decrease the gain of the input stage by about 15%. However, substituting a 12AX7 or 5751 for the upper cascode tube will have negligible effect on the input stage gain or output impedance, and may alter the sonic flavor of the circuit. 6) The measured frequency response with my Fluke 189 meter was flat between 20 Hz and 20 kHz and -1 dB at 50 kHz at 20 volts RMS output. With an additional 100 pf load (equivalent to a Stax SR007 or 009, plus the load of the meter) the high frequency response was -3dB at 46 kHz at 100 volts RMS output. 7) I built this in the spirit of a Stax Mafia Volksamp. I appreciate the fact that Kevin Gilmore thought well enough of it that he spent his time and effort to design a really nice board for it. Finally, I want to thank Kevin Gilmore for correcting a beginner mistake on the amp schemata. I built it correctly, I just drew it incorrectly. )

Gotcha. When I checked earlier yesterday it just had the basic cost without the shipping and PP fees. Paid with PayPal.

Justin, If you can send me a PayPal bill including shipping and fee to my e-mail I can pay.

I'm in the USA, New Mexico to be exact. Pay with PayPal?

Hi, Are there any left, I would like to get one pro (5 pin) jack and one standard (6 pin) jack.