EMI RS124

[letterbeacon]

Today I tried a UTC A18 transformer in place of the Peerless input transformer. It has very similar specs to the Sowter replacement with slightly lower headroom.

I hooked it up with croc clips and... I'm still getting a very thin sound! Very frustrating, but at least this means that the Peerless transformer isn't the problem.

As I did with the Peerless, I took the A18 out of circuit and hooked it up to my interface and ran some music through it and it was very tinny. So maybe it's a problem with the interface? But when I take a cable and feed the output to the input on the interface it all sounds fine.

I'm really stumped now, I can't think what it could be at all...

 

[letterbeacon]

Today I tried a UTC A18 transformer in place of the Peerless input transformer. It has very similar specs to the Sowter replacement with slightly lower headroom.

I hooked it up with croc clips and... I'm still getting a very thin sound! Very frustrating, but at least this means that the Peerless transformer isn't the problem.

As I did with the Peerless, I took the A18 out of circuit and hooked it up to my interface and ran some music through it and it was very tinny. So maybe it's a problem with the interface? But when I take a cable and feed the output to the input on the interface it all sounds fine.

I'm really stumped now, I can't think what it could be at all...

I've now tried this with an UTC A-20 and I get the same hi pass effect, so I'm thinking it's definitely not the transformer that's the issue here. I think I'm going to continue this discussion to the Magnetics forum as I don't think the problem is RS124 related.

 

[ruffrecords]

Tube inputs are very high impedance. Even with the transformer secondary not properly connected to the grid  there is usually enough capacitance for mid band signals to pass through apparently unaffected and for there to be a gradual loss below about 500Hz resulting in a very thin sound. I had this happen to me quite recently so I know it is real.

Cheers

ian

 

[letterbeacon]

But it's odd that I've had this on three separate transformers though - even when not in circuit and I'm just connecting them directly to my interface.

I've taken the discussion over to Magnetics here: https://groupdiy.com/index.php?topic=74917.0 because I think this might be off topic for the RS124.

 

[Rob Flinn]

Question:  Are you using the same cables every time you plug this into your interface ?  I've said this before, but your issue does potentially sound like you have something 1 legged somewhere.  Maybe check all your cabling & connections outside of the RS124.

 

[letterbeacon]

Question:  Are you using the same cables every time you plug this into your interface ?  I've said this before, but your issue does potentially sound like you have something 1 legged somewhere.  Maybe check all your cabling & connections outside of the RS124.

I’m using XLRs to go to the RS124. Just to rule them out I connected the same XLRs together so the interface looped back into itself and it sounded fine.

I also get the same effect when connecting the transformer via croc clips, so I don’t think it’s the cables.

It’s strange that I’m getting the same behaviour with different transformers, but heartening i suppose in that it might not be the Peerless that’s faulty.

 

[scott2000]

I’m using XLRs to go to the RS124. Just to rule them out I connected the same XLRs together so the interface looped back into itself and it sounded fine.

I also get the same effect when connecting the transformer via croc clips, so I don’t think it’s the cables.

IS The Altec an unbalanced input?

I suppose a diagram of how your croc clips are hooked up would be helpful...


edit
Looks like EmRR already brought this up in the magnetics thread.... :-[

 

[letterbeacon]

IS The Altec an unbalanced input?

I suppose a diagram of how your croc clips are hooked up would be helpful...

I think the Altec is a balanced input.

I have the croc clips hooked up as per the RS124 schematic attached - the same way with both the Peerless and the UTC A18.

edit
Looks like EmRR already brought this up in the magnetics thread.... :-[

Sorry I don't want to look like I'm cross-posting but as I thought the issue was with my transformer, I didn't want to clog up this RS124 thread with transformer issues which is why I started the new thread in Magnetics. Although now it seems like maybe the transformer isn't the issue (as I'm getting the same issue with other transformers) so it seems more appropriate for me to continue posting here.

 

[scott2000]

I have the croc clips hooked up as per the RS124 schematic attached - the same way with both the Peerless and the UTC A18.

I'm not sure what this A18 or A20 I see you tried needs connected in order to work either single ended or Push Pull but, I'd imagine center tap terminals may have to be jumped somewhere for push pull..... Maybe a detailed description of how you are testing the transformers will give more clues or not.....

I was just referring to Doug's response because it was mentioned as somewhere to consider an issue. I think it was appropriate to post there as well when trying to sort the transformer stuff....

 

[letterbeacon]

The way I have connected the transformers are as follows:

I don't have a pinout for the Peerless, so I just followed the colour on this Altec schematic: https://www.sowter.co.uk/schematics/alt436c.pdf

The pinout on the A18 is here and I connected it up the same way as the Peerless above with the CT connected.

If I get a very tinny frequency response when they're hooked up directly to my interface because of an impedance mismatch, then perhaps there's also an impedance mismatch when they're in the RS124? Should I look at the components in the circuit make up the impedance the transformer sees? If so, what would they be?

 

[Rob Flinn]

I don't think this is an impedance mismatch problem.

Maybe check the way you have the secondary centre tap connected to the pin 1's on you pot, and from them to C4/R12 junction, if this is not hooked up properly there will be problems because V1 will not be biased properly.

 

[letterbeacon]

I don't think this is an impedance mismatch problem.

Maybe check the way you have the secondary centre tap connected to the pin 1's on you pot, and from them to C4/R12 junction, if this is not hooked up properly there will be problems because V1 will not be biased properly.

Both pin 1s on the pot and the secondary CT are connected together at the same point. A wire connects that point with C4 and R12. C4's other leg goes to ground, R12's other leg goes to pins 7 and 2 on the 6AL5. That seems right to me... I think

I have the 6AL5 tube out of its socket for the time being because I just want to concentrate on fixing the amp, but I don't think that would affect things.

 

[Rob Flinn]

The thing that is flagging an issue for me is that you have the same problem with 3 different transformers that are different types.  As Doug said on your magnetics post, that is very unlikely.  Not impossible, but extremely unlikely.

 

[letterbeacon]

The thing that is flagging an issue for me is that you have the same problem with 3 different transformers that are different types.  As Doug said on your magnetics post, that is very unlikely.  Not impossible, but extremely unlikely.

Yes I'm really stumped on this one! I've built a few things in the past, but nothing that's caused me this much trouble! Thanks for your help on this so far, I really appreciate it.

I still have the high pass effect after I rebuilt the amp on turret board, which means that either a) it's an issue that's coming from a component not on the turret board - ie. a tube socket et. or b) I'm misunderstanding the schematic and rebuilt the mistake again.

 

[Rob Flinn]

One component that would affect the frequency response is C2 & C3 which should be 22nF or 0.022uF (same thing).  If you had a smaller value there you would lose bottom end.

From what I understand you have tried 2 different builds of the circuit & 3 input transformers all with the same resullts.  Using logic to me it seems likely that your issue is nothing to do with the transformers or circuit.        Personally I would probably be double checking connections outside of the circuit. 

What interface are you using  ?  Is it fully balanced ?  Are you making direct connections to the interface or are you going through a patchbay  ?  Have you verified the cables you are using with a meter on continuity ? 

 

[scott2000]

The pinout on the A18 is here and I connected it up the same way as the Peerless above with the CT connected.

Right. I was just confused how the transformers are exactly connected....jumpers etc....

Here are the A20 connections I know you said you tried isolated for example. 

 

[letterbeacon]

One component that would affect the frequency response is C2 & C3 which should be 22nF or 0.022uF (same thing).  If you had a smaller value there you would lose bottom end.

I'll double check them again, but I seem to lose bottom end immediately after transformer rather than after V1. This is the same with the Peerless, the A18 and the A20. In fact, rather than just losing bottom end it seems to boost the high frequencies too - so it's more like like a slope going from no bottom end to lots of high end.

From what I understand you have tried 2 different builds of the circuit & 3 input transformers all with the same resullts.  Using logic to me it seems likely that your issue is nothing to do with the transformers or circuit.        Personally I would probably be double checking connections outside of the circuit.

What interface are you using  ?  Is it fully balanced ?  Are you making direct connections to the interface or are you going through a patchbay  ?  Have you verified the cables you are using with a meter on continuity ?

I'm using an RME FireFace 400 interface going to a patchbay. I've checked all cables for continuity, and when I loop the output I'm using to feed the RS124 into the input on the Fireface it all sounds normal.

It seems like something is affecting the way the transformers are outputting the signal...

 

[Rob Flinn]

Take the patchbay out of the equation & plug directly from the interface to the unit.

 

[letterbeacon]

Take the patchbay out of the equation & plug directly from the interface to the unit.

Ha! That was it! I took out the patch bay and it works! I've just done a frequency sweep on it and it's flat up to about 10k where it starts to rise slightly.

I can't believe it was the patch bay! Now why would the patch bay affect it so much? When I use the same cables from the same sockets in the patch bay and loop the output to the input on the interface everything sounds fine.

Thanks for sticking with me on this Rob, I really appreciate it.

 

[Rob Flinn]

I would bet it's something to do with the way your patchbay is wired making it 1 legged.