SHP1 Amp

[Lincoln]

Finally got my SHP1 amp hitting on all 8 cylinders. Sound is great. In Tony Miller's SHP guide book he advises changing out the potentiometers R60 and R83 with 100 amp cermet potentiometer. He never mentions what watt. Does any know what watt pot. I should get or anyone have a certain brand or model I should look for.

[Ron Rich]

Lincoln,
Wattage is minimal there--1/4 W should be OK-- Ron Rich

[e093116]

Lincoln,

As a relative newbie (and also not much of an electronics tech), I've been into a couple SHP amps and in trying to source adjustable pots, I've found there isn't much out there that is reasonable, most of the items I came across were small surface mount devices that were tedious to adjust and solder to, or panel mount items which were big and considerable in their price tag.

Both of the SHP amps I've worked on would build heat almost instantly and didn't respond to adjustment, as a workaround I temporarily would install a variable resistor (aka decade box, ohm ranger, etc), turn the knob to get the reading prescribed in the manual, then solder in a resistor of the appropriate value.

Your time and money might be better spent on a decade box (if you don't have one) and a couple of resistors, compared to sourcing and installing a set of adjustable pots.

I've got about 1500 plays on my STD160 phonograph with fixed 1/4W resistors in place...no problems so far.  This past weekend my 68 year old dad even said it sounded good, he isn't complimentary about much of anything, and that was even after listening to a few turds I put in just for him (Pass the Dutchie by Musical Youth and Basketball Jones by Cheech & Chong).  Then we listened to a few good songs to make up for the torture (Don't Fear the Reaper and Magnum PI theme).

Good luck with whatever you decide and the rest of your project.

Bill

[Ron Rich]

Bill,
Do you recall what values you wound up with--is that amp equipped with the transistor , or thermistor bias circuit ?
OH--BTW--just found my copy of Tony's guide--Page 39 items "g" and "h" are wrong--should read "collector" not "E"--,
Ron Rich

[e093116]

Ron,

I think it was around 14 or 17 ohms, I forget the exact value.  I think the amp is an SHP3, not sure which version of the bias ckt it has.

I had asked some questions about a year ago on this subject, both you and Rob chided me about putting in a pot instead of a resistor...the second time around I got smarter?!

Bill

[Lincoln]

Bill,

Sounds like you've been down this road before. My plan was to order, well, I did actually order, some inexpensive pots,(the little blue square ones) and mount them on the outside of the amp. Maybe this isn't the best plan? Did you try adjusting it with the pots that were in there then measuring the resistance?

The readings I have now are 1.838v one one channel and 1.720 on the other. I guess I don't know what these mean, other than they are not within the spec listed in Tony's guide.

[Ron Rich]

Hi Lincoln,
What/how are you "measuring" ? 
Ron Rich

[Lincoln]

Per Tony's guide on page 41. Positive on q15 base and negative on q16 base. Adjust R60 to a reading of 1.65 volts +/- .05.

[robnyc]

Mar 1, 2018 5:16:33 GMT Lincoln said:

Per Tony's guide on page 41. Positive on q15 base and negative on q16 base. Adjust R60 to a reading of 1.65 volts +/- .05.

Lincoln, I idle them even lower. The one in front of me that i sue for computer audio is @ 0.4 and 0.7.

All that these settings do is keep the transistors turned on during the zero point crossover. Other wise you'll get distortion, particularly at low levels.

The original settings were far higher than needed. This seems to have been common in old transistor designs of the 1960's. I suspect it was to make absolutely sure there would never be audible crossover distortion. Cheap transistor radios and phonos were notorious for this, and it gave transistor amps a bad rep, for a while.

RobNYC

[Ron Rich]

Hi Guys,
Page 41 in Tony's SHP book does mention 1/4 watt resistors ! (just read it--honest !). As for that setting bias--be sure you are doing it, on your bench, with a cold, no input/output connected, amp. Depending on the type of bias circuit, I set them at 1.5 for the transistor circuit, or 1.7 for a thermister circuit-- IMPORTANT that you wait at least 10 minutes--heat sync should remain ICE cold--if not, you have bad outputs, and/or drivers (these should remain cold too !) Ron Rich