I guess this is what SRM-700T looks like.....

[hyperma]

 

 

I have tried every possibility to optimize the circuit, but finally STAX has chosen the worst one. I'm pretty sure that STAX has gone with SMDs, it all fine, most parts in working will not generate lots heat, also SMDs significantly reduces the distributed inductance caused by PCB board through holes.  

...... there some analyze and questions:

1, 4 large sized 7W 47kΩ resistors work as plate loads for 6SN7s, but higher value must be better, for example, two series 33K as 006T, or at least 62K 7W will far better than 47K, because 47K means too much current flow through 6SN7s, in my simulation is 9.5-10ma (grid at -9V), it's fine for 12AU7 or 12BH7, but 6SN7GT....hmmmm.

2, There are limited choice for input differential Jfets, I like using Toshiba 2SK2145 in my design, but the common sink package doesn't work here, also there are SMDs, so I guess they would better swapped to HN1C01 BJTs from Toshiba, the same thing may happened with Q6 and Q7, these components might be replaced with TTA010. Don't ask why not BC846BS, they are Japs......, local brands first as always.

3, In simulation I placed two 1815s on Q8 and Q9, but if you want to save cost and place, 2SC4944 will be better, by the way, you can also save 1 100Ω resistor due to the common emitter design.

4, The negative rail load R19 perhaps still wafer resistor in real life, which power dissipation will reach 500mW, hope they put a large size here, don't let the T2 tragedy happen again.

Extra: I didn't see any improve on original 006T or T1, there are nothing wrong with 6FQ7, the 6SN7 only consume more filament current and......in some old Chinese 12BH7s, they only swap the glass package to 9 pins, inside stuff was 6SN7 or 6N6, so in my opinion , most modern 6SN7=12BH7=6FQ7......so use 6SN7 in 700T is pointless.

[PretentiousFood]

9 hours ago, hyperma said:

Don't ask why not BC846BS

I don't think I've ever seen an SRM-700T in the flesh, but I've had pretty lousy experience with the Nexperia duals. I tried the PMP5501 and NSS40300 matched pairs as input buffers to drive a differential pair, and measured a 1/f corner as high as 5kHz on the NSS40300. Both were too noisy and had audible tape-like noise. Used in a differential pair with this much gain, I'm sure the noise is much worse. 1/f noise is generally a result of wafer contamination at the fab; maybe these other parts are made on a cleaner process, but the Toshiba parts are very quiet.

If we want JFETs, the JFE2140 is pretty compelling. Much tighter matching than the LSK parts, and a bit cheaper, too.

https://www.ti.com/product/JFE2140

 

[hyperma]

9 hours ago, PretentiousFood said:

I don't think I've ever seen an SRM-700T in the flesh, but I've had pretty lousy experience with the Nexperia duals. I tried the PMP5501 and NSS40300 matched pairs as input buffers to drive a differential pair, and measured a 1/f corner as high as 5kHz on the NSS40300. Both were too noisy and had audible tape-like noise. Used in a differential pair with this much gain, I'm sure the noise is much worse. 1/f noise is generally a result of wafer contamination at the fab; maybe these other parts are made on a cleaner process, but the Toshiba parts are very quiet.

If we want JFETs, the JFE2140 is pretty compelling. Much tighter matching than the LSK parts, and a bit cheaper, too.

https://www.ti.com/product/JFE2140

 

Yeah, recently lots people chasing the upa68ha, thinking its a perfect replacement of 2SK389 or 2SK170s pair, but it's not, most Jfets unable to provide Yfs as good as 2SK389 which is 22mSiemens, re-bias will cause cutoff too early. JFE2140 is a better choice, just bought 1 meter last night, I will try to put it on the cartridge amp see how it works.