NAD or more amp noise...

[bluesky6]

I went out and ordered several cheap used amps from GC. Cheaper than Eb*y and cheaper shipping. Goal was to try out different brands of amps and their "tone".

One Crate came in missing half the screws that hold down the speaker with the rest not tightened down. The chassis was also loosely screwed on. Sigh. Waiting to hear what GC has to say before I decide what to do with it.

A Fender Frontman Reverb Amp aka 15R looked fine and tight but had this BUZZ on power up. More buzz when I turned on reverb. Otherwise, it worked fine, if a little loud.

Touching the "ground" points didn't help. Tomorrow, I'll follow one online suggestion to connect a wire between a signal ground (at one of the jacks) to the handle hold down. If that doesn't work, that's yet another fun project! Whoopee!

My Behringer GM108 had a similar problem until I resoldered some of the power supply components (caps etc).

Can't wait for the weekend to start digging in... :duh

[J M Fahey]

Well, you KNEW what you were buying.
When I buy clothes fron the "any one $5" rack, I *expect* to have some poorly sewn button or something 

[bluesky6]

So I dug into the Frontman Reverb Amp (Frontman 15R).

It has the "newer" circuit board with a TDA2050 power amp instead of the now-obsolete uPC1188H.

Touched up some of the solder joints, replaced a cap. Then found out that the buzzing came on only when I connected the reverb wires. Without the reverb wires connected, there was hiss, but no buzz.

This is with the volume/gain/all controls turned to zero with no guitar connected.

One thing I found out during all this experimentation was that connecting a speaker cab through an attenuator (L-PAD) will actually cut off some of the noise. Presumably because the overall signal from the amp is reduced, hence the noise as well.

As a reference, I tried out the Champion 110 that was delivered today (yes, NAD again) and heard the same thing: hiss and some reverb buzz. That is probably also normal because the Champion 110 shares a lot of the same circuit as the 15R.

I see a noise-reduction project in my future. Question is: which amp should I work on first?

[Roly]

Good on you for localising it to the reverb pickup.   :dbtu:

The reverb tank frame should be grounded to the pickup end, only.  These often rely for grounding on the compression collets that hold the connector, and these can work loose and corrode.

It may be directly picking up the treble components of mains hum from a nearby transformer, perhaps the amps own.  This can only be cured by moving them apart or by adding magnetic shielding in between.

HTH

[bluesky6]

I used a bunch of aluminum foil and shielded the reverb unit. Pic attached.

Also added a ground wire from the heatsink (which is apparently the common ground point) to the foil.

All that manipulation eliminated the reverb buzz when all the controls are set to zero.

That still doesn't eliminate buzz when I turn up the reverb.

All in all, the amp is noisy. Very noticeable background hiss (without the amp, all controls to 0).

Otherwise, it plays very nicely. Especially through the G12C/S speaker on my 1x12 cab. But as I said earlier, there's some cheating there because the cab includes an L-Pad that cuts down the noise level.

I'm going to put this aside and look at the Champion 110 next. Same basic circuitry, same buzz and noise. I'll probably replace the reverb unit on that one.

[joecool85]

What is NAD?

[Roly]

What is NAD?

New Amp(Axe) Day!!!  :crazy2:

[joecool85]

Ah, makes more sense. Similar/related to GAS.

[bluesky6]

Quick update here on the Fender Reverb Amp aka first generation Mexican-made Frontman 15R.

I bought a couple of 2200uF 35V capacitors off Eb*y, swapped them in and that effectively cured the amp of any residue buzzing (noise you hear with all controls set to zero). It looks like the original power caps were faulty.

The capacitor swap also cured the amp of the cackle and spark noise coming from the speaker when I switched it off. Which was the other annoyance.

Woohoo!

I took the opportunity while the amp was disassembled to do some A/B testing of various op-amps I have lying around.

For U1, which is the clean channel input stage and originally a BA4560, I tried an NJM4580DD, TLE2072 and NE5532. The one I liked least was the TLE2072. It sounded colder/harsher. I ended up using the NE5532. The end result was a very chimey clean channel (even without the reverb) when used with my Squier CVVB.

For U2, which is the overdrive channel and originally a TL072, I tried out the OPA2132 and OPA2134. Interestingly, the OPA2134 sounded better. The OPA2132 sounded harsher and colder.

All this is subjective, obviously, with the additional caveat that I'm an inexperienced guitar player.

Other mods I did:
1. Swapped out the resistors in the input circuit around U1A for 1% metal film just because I could...
2. Reduced the power amp feedback resistor to 33kohm from 100kohm
3. Jensen C8R speaker. This last mod is one reason for the resistor change in #2 as I only had the 4ohm version of the speaker and the original was an 8ohm. The TDA20x0 family devices tend to be able to double the power output when moving from an 8ohm load to a 4ohm and I'm certain that the power transformer can't follow...

The volume controls have the same problem as all the other amps I have (other than the Vox Pathfinder 10 now) in that the living room-usable range of volume is limited. So I may yet pull the amp apart and put in a voltage divider network at the input of the power amp.