On Thursday, 11 February 2021 at 12:53:54 UTC, Scott Dorsey wrote:
gareth magennis wrote:
Right, I have this Pultec EQP-1A clone that apparently was motorboating, but only when attached to a patchbay.
Turns out the manufacturer knew certain serial numbered units were prone to this and offered a mod to fix the problem.
Customer took the unit to a tech who carried out the mod as described in an email from the company.
Only now the unit is something like 12dB down in level than before.
I have removed the mod, and there is in fact about 12dB drop in level with it fitted, the customer is correct.
Original Pultec schematic he
http://www.gyraf.dk/gy_pd/pultec/pultech.gif
The mod is to wire a 100 ohm resistor in parallel with a 0.1uF capacitor directly accross the output transformer secondary.
You sure they aren't in series? If they were in series, that would sound like
a normal zobel network designed to keep the transformer from ringing or to add
a stabilization pole for something else. In parallel they would just swamp
the transformer which is designed for a 600 ohm load.
I bet, given the description, that the thing works perfectly fine with a
600 ohm load, and only motorboats when connected to modern high-z
transformerless inputs. I bet the transformer sounds better with the
600 ohm load too. The Pultec design is intended for a 600 ohm load.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
One of the first things I did was to try them in series, in case this was the error.
This produced a significant rolloff at 10 KHz, so I guessed that wasn't the error.
The Gearslutz post says 100 Ohms in series with 25pf, which I thought made a lot more sense.
I'm reluctant to accept that without comfirmation from the company, though, which I still haven't received.
Is that a typo? should it be 22pF, or even 22nF? I struggled to find a 25pF cap.
Hence my reluctance.