Bedini-Strelioff Model 1 amplifier hum

[ddrjboo]

I acquired a Bedini-Strelioff amp at an estate sale. It was missing a few of the fuse holders, but aside from that, it looked pretty good. After obtaining some replacement holders, I was able to power the amp up.
The sound is excellent, except for the hum generated by the amp. Part of the hum seems to be from the transformer vibration and I assume there’s nothing I can do about that. The other hum comes through the speakers. It can be heard when playing at low levels but doesn’t get louder as you turn up the volume and it becomes inaudible at normal listening levels. With blank RCA connectors hooked up to the amp the hum is MUCH lower. I’ve tried 3 different sources (2 preamps and my phone) and the hum is about the same for all 3.
Is it possible that the issue is the main filter caps? These caps are humongous 39000 uF, 75 V. See attached pic. I hate to replace them if they aren’t the problem because it will be expensive. Is it possible it’s just normal for this amp and there’s nothing to fix?
I would appreciate opinions and advice. I can live with the hum if it’s normal, but would like to eliminate it if possible.

 

[century tek]

Since it is in both channels, I would probably be looking at the power supply capacitors. If it's capacitor coupled, those big one's are usually not the cause of hum.

 

[ddrjboo]

Thanks for your reply. Without a schematic I am a bit lost. Each channel has a pull out module like this.

There are 3 electrolytic caps under this board marked 47 uF 63 V. They each test at 60 uF in circuit with my esr meter. You may be able to pick them out here

There is some minor heat discoloration on board shown here. Resistors are on the other side of the board

There is a 600v cap shown below that measures 0.01 uF. I don’t know what is beneath the black wrapping to the left

The board shown above is connected to the heat sink, and all of the transistors have thermal paste to give them contact with the heat sink. Pulling the board up seems like a waste if I’m just shooting in the dark.
Anyone previously own one of these that can say if the hum is normal?

 

[Binkman]

G'day I can't find "model 1" is this the amp?
https://audiokarma.org/forums/index.php?threads/have-you-ever-seen-a-strellioff-dc1.575518/
Throwing darts here...
Is the hum audibly loud?
Is there a ground lug on the ac cord?

Is there a ground lug on the chassis? wondering if a ground wire attached to the chassis and the pre chassis would eliminate the hum. even if no lug you could try that.

 

[ddrjboo]

Good day! The model may be referred to more commonly as a “200/200” as that is more prominent on the front of the amp.

There is no ground connection on the amp to ground to the preamp. It has a 3-prong plug, but I have tried using a 2-prong cheater plug with no improvement in the hum.
I’ll have to put it back together to try grounding the preamp to the amp chassis. If I try that should I use the 2 or 3 prong connection? Or try both?
Edit: I tried grounding the preamp to the amp and the hum increases slightly.
The amp has two quarter inch inputs as well as the RCA inputs. Is it possible the hum is from the extra inputs? I know unused phone inputs can cause hum.

 

[ddrjboo]

I have looked at photos of the Strelioff DC1 amp and they look very similar. They seem to have same main filter caps and the same modules.

 

[century tek]

Let's see an overall picture of the amp. The power supply would be separate.

 

[ddrjboo]

All I see is fuses, a transformer, a relay, and wires aside from the 2 modules.
From the top.


And from the bottom

 

[hamrules]

I acquired a Bedini-Strelioff amp at an estate sale. It was missing a few of the fuse holders, but aside from that, it looked pretty good. After obtaining some replacement holders, I was able to power the amp up.
The sound is excellent, except for the hum generated by the amp. Part of the hum seems to be from the transformer and I assume there’s nothing I can do about that. The other hum comes through the speakers. It can be heard when playing at low levels but doesn’t get louder as you turn up the volume and it becomes inaudible at normal listening levels. With blank RCA connectors hooked up to the amp the hum is MUCH lower. I’ve tried 3 different sources (2 preamps and my phone) and the hum is about the same for all 3.
Is it possible that the issue is the main filter caps? These caps are humongous 37000 uF, 75 V. See attached pic. I hate to replace them if they aren’t the problem because it will be expensive. Is it possible it’s just normal for this amp and there’s nothing to fix?
I would appreciate opinions and advice. I can live with the hum if it’s normal, but would like to eliminate it if possible.

Have you turned it on hooked up to the speakers with nothing hooked to the inputs and did it then also hum thru the speakers?

 

[ddrjboo]

Have you turned it on hooked up to the speakers with nothing hooked to the inputs and did it then also hum thru the speakers?

Yes. With nothing connected to the RCAs there is about the same amount of hum as with a preamp. With RCA blank connectors the hum is much lower.

 

[ddrjboo]

I removed the center face plate to see if any more circuits were hidden there, but all that’s back there are the LEDs and the board for them.

 

[Binkman]

With RCA blank connectors the hum is much lower.

This is a good 'hint'. Check those neg. rca connections and perhaps trace back if soldering is good. perhaps shielding required.
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I've only read of one amp on a.k. (can't remember the model) that used a 3 prong p/cord and the hum was eliminated with/out the ground but noted this amp was already chassis grounded w/o the ground plug. However w/the ground this amp could be hum free. weird mfg. eng. change perhaps.

 

[ddrjboo]

The amp is wired so that the RCA inputs are wired to the 1/4 inch inputs. The wires that go from the 1/4 inch inputs to the rest of the amp are shielded. The wires from the RCAs to the 1/4 inch aren't shielded, but they are a short run and twisted together. It's odd because if I connect one blank RCA connector to either input with the other RCA plug connected to the preamp, there is NO hum. Almost all of the connections are spade connectors like these, so not much soldering.

I will check where the modules are connected to the 1/4 inch input to see if they are soldered, but this requires some disassembly.
.

 

[hamrules]

The amp is wired so that the RCA inputs are wired to the 1/4 inch inputs. The wires that go from the 1/4 inch inputs to the rest of the amp are shielded. The wires from the RCAs to the 1/4 inch aren't shielded, but they are a short run and twisted together. It's odd because if I connect one blank RCA connector to either input with the other RCA plug connected to the preamp, there is NO hum. Almost all of the connections are spade connectors like these, so not much soldering.

I will check where the modules are connected to the 1/4 inch input to see if they are soldered, but this requires some disassembly.
.

Sure seems like some kind of ground issue in the amp. Wonder if the 1/4” jacks would cause the hum also.

 

[ddrjboo]

Sure seems like some kind of ground issue in the amp. Wonder if the 1/4” jacks would cause the hum also.

That's a good question. I have some old 1/4 inch male plugs that I should be able to connect to a cable with RCA connectors to give it a try.

 

[ddrjboo]

I didn't notice any difference in the hum after connecting the preamp to the amplifier with my homemade RCA to 1/4 inch plug connectors.

 

[ddrjboo]

I re-soldered all of the connections on the molex connectors that go to the 2 pullout modules and no improvement. I did manage to find the bridge rectifier. Here it is with the blue wires connected to it:

 

[Todd R]

Pardon me for jumping in here, but a friend brought over a Bendini / Strelioff Model 1, 100/100 for me to look at and see if it's working. The power cord appears to have a bad repair that I plan to fix, but...
Where is the power switch for this amp? I'm baffled.

 

[ddrjboo]

Good question! There is no power switch. You plug it in and the relay pops, and hopefully everything works. If that bothers you, you can plug into a power strip and use the power switch on the strip.
I have had fun asking people to plug in my Bedini/Strelioff and watching their reaction when the relay clicks in.
The thing that I find most difficult to work on this amp is that the modules have to be removed to take any kind of measurements, that and the fact I can't find a schematic. Aside from a hum at low volume, mine sounds fantastic.