Matamp Minimat 2 - accidental attenuator discovery

victorludorum

The attenuator works on the 8ohm speaker outlet, but mine is currently connected to a 16ohm Jensen C12N. This amp has a very strong mid voice and not much bass, but by turning the attenuator fully on, it seems to act as a bass boost, and does seem to cut the volume a little too. Not so great for cleans, but excellent for fuzzier leads. Maybe it's my hearing, but it seems that it's clearly having an effect on the 16ohm output. Does this sound normal?

ICBM

If it's wired wrong it wouldn't be the first Minimat I've come across like that...

victorludorum

It's the 2nd version. Is it likely to be damaging the amp in any way?

ICBM

victorludorum said:

It's the 2nd version. Is it likely to be damaging the amp in any way?

No, it just may not be sounding as good as it should. It sounds like the attenuator may be partly engaged even when it shouldn't be.

On the one I worked on, the 8 and 16 ohm taps of the OT were connected to the wrong jacks, so when you thought you were matching it right, you were matching it wrong! So both outputs were a little quiet and sounded odd in opposite ways... which was the clue.

victorludorum

Calling @Van_Hayden ; Do you have any thoughts on this? Is it worth me taking a photo of the guts?

victorludorum

@ICBM I imagine that there isn't, but Is there any way to test the speaker outs to see if they're wired correctly? I ask as I have been playing it through the 8ohm out into a 16ohm speaker, and it sounds way better to me; much fuller. When I run it through the 16ohm out it loses a lot of bass and sounds much thinner, so I'm asking myself if there wiring is wrong. I can post a photo of the guts, but the wires are a little bit obscured by the PCB.

Kitsune

Do they still make Minimats? I wanted one really really really badly about 10-12 years ago, and now I have the money they don't seem to make them any more!

victorludorum

They do still make a version I believe, but it is the mk3 one with simpler controls. There are 2 for sale on ebay at the moment. One overpriced head, and one well priced combo.

ICBM

victorludorum said:

@ICBM I imagine that there isn't, but Is there any way to test the speaker outs to see if they're wired correctly? I ask as I have been playing it through the 8ohm out into a 16ohm speaker, and it sounds way better to me; much fuller. When I run it through the 16ohm out it loses a lot of bass and sounds much thinner, so I'm asking myself if there wiring is wrong. I can post a photo of the guts, but the wires are a little bit obscured by the PCB.

I can't remember the colour coding anyway, so it won't help.

Do you have a mulitmeter? If so, measure the resistance of the outputs to ground - they will be very low and there will be very little difference (eg 0.3 ohm and 0.4 ohm), but the higher of the two is the 16-ohm winding and the lower the 8-ohm tap.

From your description it does sound like a possibility that they're the wrong way round!

victorludorum

It seems that you're right and they're wired wrong. I'm getting slightly erratic readings as I'm using the chassis as earth, but the 16ohm out is coming in lower than the 8 ohm socket. It all makes sense now why i was never really happy with it until recently, when I plugged into the 8 ohm output to a 16 ohm speaker. It also explains that quirky attenuator behaviour I believe. ]

Thans @ICBM You're a gent!

victorludorum

I've just tried to test the readings again and I'm only getting erratic readings on the output that has the attenuator connected. Moving the attenuator seems to throw things completely, and not in a way that I'd expect. Surely increasing the attenuation would increase the resistance, but the readings don't suggest that, even though the attenuator performs as it should when playing.

I'm getting a clear 0.5 on the 16 ohm output, but the 8 ohm one is giving readings that are far higher (erratic readings from 8-12 that change constantly)

victorludorum

Calling @ICBM Here are some photos. Are there any indications there or are they too limited?

http://www.victorludorum.org/P1120439.JPG
http://www.victorludorum.org/P1120440.JPG
http://www.victorludorum.org/P1120441.JPG

ICBM

Sorry, it doesn't help much without knowing the colour coding. The best thing to do is disconnect the two transformer wires which aren't connected to ground - red and yellow I think - and measure their resistance to ground.

victorludorum

I called the chaps at Matamp who advised me to disconnect the attenuator, which I have now done. It sounds better to me and has allowed me to measure the resistance levels correctly, and the readings confirm that it is wired correctly. The attenuator might be faulty, but to be honest I never use it, so I'm happy to leave it disconnected.

Cheers for your help with this, it's appreciated as always ;-)