Picked up another p1000 used and its got seriously noisy gain pots. Did the basic cleaning and they're better but noise is in and out now.
Can anyone point me to some decent replacements? Doesn't seem to be much for info floating around.
Can anyone point me to some decent replacements? Doesn't seem to be much for info floating around.
Hello Cyrus_s,
I'm having a similar experience with a Hafler P1000 and this is what I've found:
https://www.bourns.com/docs/product-datasheets/ptd90.pdf
I haven't received my pots yet but they should be delivered in the next two or three days. I'll see if they fit and report back.
Regards
Giorgio
I'm having a similar experience with a Hafler P1000 and this is what I've found:
- The P1000 manual, including schematics and part list is available at:
- The gain pot R24 is reported as 10k Pot, Dual. No MFR P/N, no info about its curve.
- From inspection, the component is marked as B10k, which seems to suggest a B curve, or Linear.
- A quick check on Digikey and the Bourns website suggests the PDB182K415K103B as a possible replacement, see:
https://www.bourns.com/docs/product-datasheets/ptd90.pdf
I haven't received my pots yet but they should be delivered in the next two or three days. I'll see if they fit and report back.
Regards
Giorgio
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Hi,
The PTD pot is much smaller than the original part but should be possible to adapt it to fit into the PCB. The shaft is also shorter and a 20mm shaft is probably right.
I hope this helps
Best regards
G.
The PTD pot is much smaller than the original part but should be possible to adapt it to fit into the PCB. The shaft is also shorter and a 20mm shaft is probably right.
I hope this helps
Best regards
G.
Not sure why a potentiometer on a power amplifier should wear out. They hardly get used compared to a preamp or integrated amp. Potentiometers don't like DC voltage on them, however, and that is the normal source of crackling on rotation. I use DeOxit P5 for cleaning pots and it works really well, but it won't stop noise from DC across a pot. What is the DC voltage across R24B, namely pins 4 & 5? Ideally this should be close to 0mV.
The Hafler uses a DC coupled U1 TL072C to drive R24B. The TL072C have a poor spec for DC offset, and they don't rate in noise and distortion performance compared to more contemporary op-amps for audio either. I'm not qualified to recommend a replacement, but maybe someone else can.
Another possible source of offset is the input coupling capacitors to U1, C23 & C24, which are 0.47µF 50V. There's no sense in such a small value being electrolytic so I would replce with a film capacitor eg WIMA MKP4 type. The load impedance on the output of this cap is very high so there's not much point in a larger value.
The Hafler uses a DC coupled U1 TL072C to drive R24B. The TL072C have a poor spec for DC offset, and they don't rate in noise and distortion performance compared to more contemporary op-amps for audio either. I'm not qualified to recommend a replacement, but maybe someone else can.
Another possible source of offset is the input coupling capacitors to U1, C23 & C24, which are 0.47µF 50V. There's no sense in such a small value being electrolytic so I would replce with a film capacitor eg WIMA MKP4 type. The load impedance on the output of this cap is very high so there's not much point in a larger value.
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Hi johnmath,
Points well taken but I doubt the DC offset of a unity gain amplifier is enough to make a pot noisy. Unless there is something very wrong with it (one supply rail way out of specs, etc.).
As for OpAmp replacement, I would consider the OPA2134 as a first candidate and please note this is a relatively old part, although not as old as the TL072. The TI website suggests to look into their OPA1656 for a suitable replacement (of the OPA2134). In any case, I've never felt the urge to replace the TL072 in the (many) P1000 I have. To my old ears, they sounds good as designed. I'm quite sure they can be improved but the elegance of the design would be compromised.
Regarding the input caps. they look like electrolytics in the schematics but they look like mylars (or similar) in the component placement figure. Unfortunately, Hafler used the same symbol for any type of capacitor. The BOM is actually listing different types and C23 and C24 are not listed as electrolytics.
I will re-open my faulty P1000 as soon as I can and will report back. In the meanwhile, I've found better pot replacements from Mouser. From inspection, the open frame design of the pot, along with the design of the amplifier enclosure with vents, seems to indicate wiper contact wearout or contamination as the main culprit but I wouldn't speculate further. I'll bring my amp in the lab and see for myself (sooner or later, don't hold your breath 🙂)
Points well taken but I doubt the DC offset of a unity gain amplifier is enough to make a pot noisy. Unless there is something very wrong with it (one supply rail way out of specs, etc.).
As for OpAmp replacement, I would consider the OPA2134 as a first candidate and please note this is a relatively old part, although not as old as the TL072. The TI website suggests to look into their OPA1656 for a suitable replacement (of the OPA2134). In any case, I've never felt the urge to replace the TL072 in the (many) P1000 I have. To my old ears, they sounds good as designed. I'm quite sure they can be improved but the elegance of the design would be compromised.
Regarding the input caps. they look like electrolytics in the schematics but they look like mylars (or similar) in the component placement figure. Unfortunately, Hafler used the same symbol for any type of capacitor. The BOM is actually listing different types and C23 and C24 are not listed as electrolytics.
I will re-open my faulty P1000 as soon as I can and will report back. In the meanwhile, I've found better pot replacements from Mouser. From inspection, the open frame design of the pot, along with the design of the amplifier enclosure with vents, seems to indicate wiper contact wearout or contamination as the main culprit but I wouldn't speculate further. I'll bring my amp in the lab and see for myself (sooner or later, don't hold your breath 🙂)
Hi,
I took a peek at the PCB of my P1000 and C23 and C24 are definitely mylars.
One other thing that may be noisy is SW3, if you are experiencing noise when using the RCA input. Like R24, it is an open frame design, thus susceptible to contact contamination.
I took a peek at the PCB of my P1000 and C23 and C24 are definitely mylars.
One other thing that may be noisy is SW3, if you are experiencing noise when using the RCA input. Like R24, it is an open frame design, thus susceptible to contact contamination.
My mistake. The schematic uses a symbol that is normally associated with electrolytic capacitors, but the inventry does not indicate they are.
DC on a pot is what normally causes excess noise. Over time the DC current through the potentiometer causes anodic oxidation and/or migration of the metal on the wiper contact increasing the contact resistance and increasing the wiper noise. It's worse if the input to the pot is negative relative to the common. Replacing the potentiometer will reduce the DC induced noise (not eliminate it) but over time it will come back as the electrolysis recurs.
Its a bit more subtle than that. Pots should work the same whatever the signal, and in general they do this, DC or AC are the same.
However if the DC input bias current for a transistor or opamp comes from the wiper connection of a pot, then any intermittency of contact of that wiper during operation will interrupt bias current for the active device, causing it to widely veer from normal operating conditions (ie slam its output towards one of the rails). Even extremely brief periods (measured in ms) of intermittency will become huge transients (12V or more in standard opamp circuitry).
If the device has a separate DC bias path and there is a capacitor on the pot's wiper, then the intermittency will only cause signal-level drop-outs, which is orders of magnitude less audible (but can still be an issue with a well worn pot).
For FET input devices the DC bias currents are so small that stray capacitance will reduce the effect of a scratchy pot dramatically - for instance with stray capacitance around 10pF and 10pA of bias current there is a low-pass filtering of transients limiting the slewing to 1V/s, rather than something more like 10V/ms for bipolar inputs (which often have 100nA or more of bias current).
The practical upshot is always use a wiper capacitor if it leads to a bipolar device, providing a separate path for DC bias.