Building a symmetrical PSU B1 buffer

[pchw]

Just put a couple of pins for R1 to solder various values on, and reduce by paralleling until you will reach Vdrop across R1/R1=62mA to match the other half. Its obviously VGS related.

LOL, I already prepared 2 extra pins for any hotrod mods in the future 😀 They now come in handy. I also put it a couple DIP8 socket in the places of the 2 100uF LED bypass caps for future cap experiment. Yeah, I did do my homework before I started my build 🙂

 

[Salas]

He, he, a futuristic DIYer! 🙂 Will be happy to see your pics and impressions here in the end. I will see to make those 2 mods integral if future GB will ever run. Those two mods, heavy copper for hotrod current, and plated holes I think will make it better enough for a ''collector's edition'' final small run. Though the normal boards still do the same stuff with a little work.

 

[ra7]

I have GR grade 2sk170s... they have Idss values spread all over the place... from 3mA to 16mA.... I don't know if these are fake, but they look poorer in appearance than the ones I bought from Spencer on diyaudio. Also, Spencer's BL grade JFETs measure exactly around 8-10mA.

Now, I also ordered some BL grade from Mouser. They all measured over 15mA Idss.... I don't know what's going on. I'm using the JFET testing method in Salas's NJFET pdf.
I've triple checked my connections. I don't think there's anything wrong there.

 

[Salas]

No you aren't wrong. Fake GRs and mixed BLs with Vs. If you have enough 8-10mA from Spencer use those. Most secure option.

 

[ra7]

Do you think the Mouser ones are fake too? They say BL grade but all measure over 15 mA.

 

[Salas]

No. I would think that the factory mislabeled some V for BL and sent em out. You don't miss out, V are sought after for head amps.

 

[matt09]

Thanks Sales, I think I'll stick with this design and use seperate supplies.
I see you mentioned using 2x68r resistors for enough current for use without heatsinks.

I am needing 4 channels so is this enough and will it work without degrading performance to drive 2 sets of preamp's with this, or will I need to increase the current with 3x68r, or even double to 4x68r?

Thanks

 

[regiregi22]

BL grade ranges between 5 and 9mA IDSS, but lower grade will work too

 

[Salas]

matt09:

To answer that I should know the current demands. If you mean 4 DCB1 channels then its about +/- 40mA demand, use 3x68 to reach 100mA. The amount to burn on the shunt elements remains the same, maybe you would need a makeshift aluminum small rectangular plate or a small TO-220 clip on sink for the CCS Mosfets. If you drill it and mount it backwards maybe.

 

[matt09]

Thanks Salas, yes I meant 4 DCB1 channels each driving an lm318 on the myref design.
I assume you were refering to TO-220 just as a heatsink size as the IRPF240's arent?

Thanks

 

[Salas]

Yes, you know there is some variety of small off pcb ''wing'' light mass sinks like those they clip on LM317s. Some have only a clip, some only a hole, some both. Just to aid the CCS Mosfet that will carry the whole current all the time. But even a rectangular aluminum cut out will do. Is there capacitive coupling between the DCB1s and the input to LM318s?

 

[matt09]

I wasn't going to put any where they aren't necessary because the power amp output has a power on relay delay. I think I'm good to go with a single cap at each respective input to the dual preamp setup I'm using?

 

[Salas]

If the amp has output protection, then its a fortunate situation. Just measure its output on a dummy and see there is no DC during on/off (and for a little while after off) when all B1 driving is integrated. Must be switched from a main rocker as a whole. Does it sense DC also? If it does and its reliable, then you can rely on it even if one DCB1 rail ever fails and gives DC on output. If not, better ac couple to LM318s.

 

[matt09]

Yes it does, the audiophile design chip amp by Maurio is the best fully featured design in that forum probably. Could I even go with no caps at all as there is no dc offset at switch on from my source?

Thanks

 

[Salas]

If the output protection will reliably work always, Its a calculated risk for you to take. Are the speakers expensive?

 

[matt09]

Not hugely both sets are under £500 in total, I'll think it over

My transformer for the superregs with the 318 is a little high for this circuit in voltage, I wont lose anything in preregulating with a zener or 317/337 to attain the right dc voltage for the pre will I?

Many thanks

 

[Salas]

More circuitry is more circuitry. But its a solution. +/-17V is what needed for DC in to the shunts. 317s need 2.2V minimum headroom not to drop out regulation, remember.

 

[regiregi22]

Matt, what have you thought for controlling volume? I am going too with a DCB1 (already built) and a MyRef_C, will be using a Lightspeed attenuator before the DCB1.

 

[wintermute]

More circuitry is more circuitry. But its a solution. +/-17V is what needed for DC in to the shunts. 317s need 2.2V minimum headroom not to drop out regulation, remember.

True, but the Datasheets quote Vin - Vout >= 3V for the rated load regulation specs (as well as others) so that to me implies you need to allow 3V margin for them to operate "within spec" even if the dropout margin is lower... certainly I found recently when simulating for the LM317 circuit I'm designing that I needed a higher voltage than I thought I would to get the results I expected.

So to be on the safe side (and to make sure your LM317 is actually being of some use), I'd add an extra 3V for your LM317 on top of what the shunt needs 🙂

Tony.

 

[AndrewT]

you also need to add the ripple coming from the PSU. In fact you measure the average and you actually need to add half the ripple peak to peak to find the lowest voltage in the trough of the waveform.