Forté Audio Model 3 Hum

[dastinger]

Vdc readings are okay - showing +/-39Vdc rails and +/-78.1Vdc rails

Vac readings are suspicious
you had 200mVac or 2Vac range set on DMM?

indulge me, and repeat that Vac measurement

it seems as - if both channels are same or approx same heatsink temperature and if hum is present in both channels in same amount - that these big caps are suspicious

of course - all contacts (cap screws, fuse holder snap on connectors etc) must be clean and tight ....

I actually did think about the ACV/VAC range because my multimeter does not have 200m nor 2. It has 750 and 200. I have another multimeter, but it doesn't read voltage. Here's the one that I used (I measured on ACV 200). The DCV was measured on 1000:

Is this fine or should I try getting another one?

I didn't check the contacts. I assumed that since the amp was serviced, things like that were checked, but I will do it once I get inside again.

The amp sounds great even measuring like this, but there's something definitely wrong (hum and these measurements). I wonder how it may sound when everything is right.

When I am debugging hum, I want to eliminate ground loops as a source.

The way to do that is to use a battery powered device known to not have any 60Hz mains hum in it. Like your smartphone with a 3.5mm to RCA jack.

That is your reference “quiet and ground loop free source”. It may not be the best sound quality. But it is almost guaranteed to be hum free.

Not connecting anything to the input of a power amp may not be a reasonable mode of operation. What purpose does a power amp have when not connected to an input. Some amps will hum with no input. Grounding the input is better. Make a special RCA grounding plug.

Next thing is to work on one channel at a time. Ground one channel input. Connect the RCA from the phone 3.5mm to RCA adapter to the other channel.

If there is hum, use a DVM in AC mV mode and measure the noise. Something > 1mV rms will genenerally be audible.

If it is quiet with one channel connected to the phone source, you are in luck. You have a ground loop problem and not an intrinsic amp noise problem.

Now connect the other RCA to the amp, this will make a common contact between the RCA shields and sometimes, hum will now appear if the amp has a ground loop issue.

For the case that there is hum with both RCA inputs grounded, then the hum is intrinsic to the amp and there is a deeper issue.

Solid state amps should not have audible hum if well designed. As this was a commercial product designed by Mr Pass, I would assume it to be hum free as designed.

Some sources of hum have been mentioned above.

1. DC in your AC mains. This saturates the transformer and can result in hum. This can be addressed with a DC blocker circuit for the mains. Many DIY designs exist to fix this. A couple of big back to back caps and a string of power diodes are generally the parts needed.

2. Non sinuisoidal waveform on your mains (mine look like a series of breadloaves on a conveyer belt) - this generated harmonics in the mains and may pass through as hum.

3. The ground strap or bonding wire from the chassis to the heatsink or protective earth connection on the amp PCB is faulty or not connected to the IEC mains “earth ground”.

4. Failing old electrolytic caps.

5. Some other electrical issue - will require more debugging by experts familiar with the particular amp.

The loud mechanical hum you hear for 2 seconds is the inrush current mechanically moving the trafo at power up due to magnetorestriction and it goes away after the inrush is gone. But not completely. That might be the DC or the non sine AC mains.

A good amp should not have speaker outputs with more than say 0.2mV to 0.4mV rms noise. Ideally close to 0.1mV or less.

I hope that helps.

Thank you so much for the detailed post. I'll go over this step by step tomorrow or on the weekend. It's a lot to take in

Just to clarify, you said "make a special RCA grounding plug". So, would a 3.5 jack -> RCA normal cable work connected to a smartphone?

Then you said:

Connect the RCA from the phone 3.5mm to RCA adapter to the other channel.

So, connect the 3.5mm jack to the phone and only one RCA either to the left or right channel? Leave one disconnected and the other connected and check for hum. If there is hum, disconnect and try the other one. Is that it?

If it is, I'll be able to do this rather quickly because I already have a cable ready (again, it's a normal run of the mill jack 3.5 -> RCA cable. Hope that's it).

Again, thank you guys!

 

[Zen Mod]

with that DMM, do not even waste time having mentioned Vac measurements

- they're bad and not particularly precise, but
-main reason is that Vac range it have are simply too high (not enough resolution)

 

[dastinger]

Alright, I'll get a new one. Do you happen to have any recommendation? Something that's just good enough and doesn't cost an arm and a leg. I'm in Portugal, but maybe I can get it through Amazon. If it's available on Amazon.es I can get it this week still and we may get going with testing.

 

[Zen Mod]

I believe best thing should be to find some other Tech. , able to check (in various ways) are those big caps good or not

all symptoms and simple logic are leading to conclusion that they are the culprit

 

[dastinger]

I believe best thing should be to find some other Tech. , able to check (in various ways) are those big caps good or not

all symptoms and simple logic are leading to conclusion that they are the culprit

I understand what you're saying, but thing is, I know I am able to test this amp and replacing caps isn't that hard as long as I'm able to buy good ones.

I know that with the help of this forums I'd be able to save A LOT of money. We don't have that many techs here and I'd spend a pretty penny for their work + the capacitors when I could most probably do it myself.

If what it takes to move on is getting a decent multimeter, I will. Once I do, I'll measure the amp again and let you know. No one learns by staying put, right?

 

[Zen Mod]

If you're willing to do it by yourself, I'm even happier

see here, Uni-T, take whichever you want, just avoid those with probe wires hardwired to meter itself (PITA to change when they break) :

https://www.amazon.es/s?k=uni-t+mul...MÅŽÕÑ&qid=1634770666&ref=sr_st_price-asc-rank

 

[xrk971]

Alright, I'll get a new one. Do you happen to have any recommendation? Something that's just good enough and doesn't cost an arm and a leg. I'm in Portugal, but maybe I can get it through Amazon. If it's available on Amazon.es I can get it this week still and we may get going with testing.

I use a Fluke 101 and find the mV AC setting very good for measuring amp output noise. They are about $45 to $70. I use mine literally 10 times a day for the past 6 years.

FLUKE-101 Digital Multimeter https://www.amazon.com/dp/B00HE6MIJY/ref=cm_sw_r_cp_api_glt_fabc_CJGRYMYF9QYZ4YG45HB2

By making a special RCA grounding plug, I mean take an RCA jack like a cheap one you get as accessory with certain cheap HiFi stuff and cut the Jack off the cable leaving a few inches of cable, strip the wires and solder the center to the shield to ground it. Now you have a grounded RCA plug that can be used to short the inputs of amps.

Or buy blank RCA jacks and connect a jumper from center to ground.

Honbay 20PCS Solder RCA Plug Male Audio Video Adapter Connector Professional https://www.amazon.com/dp/B01FJBA5YU/ref=cm_sw_r_cp_api_glt_fabc_CPFCJ8Z49X62DESMC4YQ

 

[dastinger]

If you're willing to do it by yourself, I'm even happier

see here, Uni-T, take whichever you want, just avoid those with probe wires hardwired to meter itself (PITA to change when they break) :

https://www.amazon.es/s?k=uni-t+mul...MÅŽÕÑ&qid=1634770666&ref=sr_st_price-asc-rank

Thank you, I'll go ahead and buy one either locally or on Amazon. I'll then do what XRK mentioned and do all the readings again. We'll get to the bottom of this

I use a Fluke 101 and find the mV AC setting very good for measuring amp output noise. They are about $45 to $70. I use mine literally 10 times a day for the past 6 years.

FLUKE-101 Digital Multimeter https://www.amazon.com/dp/B00HE6MIJY/ref=cm_sw_r_cp_api_glt_fabc_CJGRYMYF9QYZ4YG45HB2

By making a special RCA grounding plug, I mean take an RCA jack like a cheap one you get as accessory with certain cheap HiFi stuff and cut the Jack off the cable leaving a few inches of cable, strip the wires and solder the center to the shield to ground it. Now you have a grounded RCA plug that can be used to short the inputs of amps.

Or buy blank RCA jacks and connect a jumper from center to ground.

Honbay 20PCS Solder RCA Plug Male Audio Video Adapter Connector Professional https://www.amazon.com/dp/B01FJBA5YU/ref=cm_sw_r_cp_api_glt_fabc_CPFCJ8Z49X62DESMC4YQ

Gotcha, that's pretty easy! I just need to buy the multimeter and I'll get to work. As soon as I have everything done I'll let you guys know.

I also need to get my old Bose speakers from the garage. I don't want my B&W connected to the amp for this in case something goes wrong.

Thank you so much!

 

[dastinger]

The UNI-T UT131A would work just fine, correct? They have the specs on their website (I didn't link it because I'm not sure if that's alright). If so, I'll have it by saturday and get to testing.

Also, @xrk971 I have 470ohm 0.6W resistors and spare RCA plugs laying around. Would it be best to build the RCA grounding plug with the resistors or the cable only is fine?

 

[Zen Mod]

that DMM is fine for your needs

 

[chede]

Also, @xrk971 I have 470ohm 0.6W resistors and spare RCA plugs laying around. Would it be best to build the RCA grounding plug with the resistors or the cable only is fine?

I'm not xrk, but you want your shorting plugs "really short". Either use a piece of hookup wire, nothing fancy, or a lead clipping from a resistor - I use resistor clippings all the time. My preferred Yageo metal film resistors have tinned, non-magnetic leads that are fine for all kind of hook-up or bridging purposes.

Regards, Claas

 

[dastinger]

that DMM is fine for your needs

Thank you sir! Saturday I'll have news.

I'm not xrk, but you want your shorting plugs "really short". Either use a piece of hookup wire, nothing fancy, or a lead clipping from a resistor - I use resistor clippings all the time. My preferred Yageo metal film resistors have tinned, non-magnetic leads that are fine for all kind of hook-up or bridging purposes.

Regards, Claas

Wait, so you just cut off the leads from the resistors, solder one tip of the lead to the center and the other to the ground of the RCA plug and that's it?

 

[Zen Mod]

yup

 

[dastinger]

Gotcha. That's what I was gonna do with regular wire, but since I have the resistors, I'll use the leads. Thanks!

 

[chede]

 

[xrk971]

We have buckets of resistor clippings lying around. I save them exactly for this or to make small P2P builds.

 

[dastinger]

Ahahah some people have pens on their desks. You guys have resistors' clippings.

 

[dastinger]

OK guys. So, I got the multimeter and I already created the shorting RCA plugs. I have two questions before I get into it.

I mentioned not wanting my BW speakers connected to the amp, but I'm worried that the other ones I have are not sensitive enough to pick up the hum.

1 - Is it dangerous to do this with the speakers connected?
2 - If it is, I'm willing to test it with the old speakers but they're 120W (Bose 201 Series V) and this amp has 200W per channel. Will I blow the speakers? Remember that I'll be plugging my phone directly to it so the volume control will be on the phone. I don't even need to play anything so no sound would actually come out of the speakers, but better safe than sorry.

I searched but some say yes, some say no. I trust you guys more to answer that.

Thanks

 

[Zen Mod]

there is no need of having speakers connected, for measurement of ripple

 

[dastinger]

there is no need of having speakers connected, for measurement of ripple

I'm mainly asking because of what XRK told me to do. The short + phone connected to the other input.

But I may do the readings in the mean time without doing that. Do you want me to short both inputs for the readings or power only with nothing connected to the inputs is fine?