Soldano SLO100 Efects Loop

[Deadpan]

Been struggling with my SLO lately and found the issue seems to be related to the effects loop. With the loop off it doesn't sound great. I have to have the loop on and at max for the best sound. The power tube bias is right at spec.

Anyone have experience with this?

 

[headcrash]

Is it an old SLO or a new one? Do you test this with just a patch cable in the loop?

 

[Deadpan]

Is it an old SLO or a new one? Do you test this with just a patch cable in the loop?

This is a new SLO.

I will test some.

 

[headcrash]

You should test the loop with a short patch cable only, and set the level control on the back of the amp, so that the volume with loop on and off is the same. I assume the loop is fully transparent, so IF then there is still a change in sound, maybe the tube for the FX loop is faulty, or you are even saturating the buffer stage(s) of the loop. Why?

Keep in mind, that the level you feed into the FX is determined by the master volumes of the front of the amp, when the loop is engaged. The overall volume of the amp will then be controlled by the level pot on the back of the amp (I consider the design of the loop and how it works to be bad practice, but it is how it is).
So, when you feed too high a level into your FX units you might drive them into saturation, depending on how high your signal levels are, and which FX units you use. It may also be possible, that you drive the tube buffer stage(s) into saturation.
When you don't find any tone alteration using that patch cable in the loop, you might check, how high you can set the master volumes, before you hear any saturation effects of the buffer stages themselves (if any at all). This can always be done by switching the loop in and out, which of course will introduce tremendously loud volumes. So prepare your neighbors...

 

[Deadpan]

So, when matching the level of the loop off with the loop on no patch cable (the level still effects the volume) it's pretty transparent. Adding a patch cable does increase the volume a small amount.

With the loop on and the loop level at max, with or without a cable, sounds more open and less congested. Granted it is a bit louder than with the loop off but raising the channel master to compensate only makes the congested sound louder.

From what I see the older SLOs didn't have a defeatable or variable level effects loop?

It seems that adding these options have this effect?

When I received the amp I always had an EQ in the loop so I didn't notice this.

 

[MirrorProfiles]

So, when matching the level of the loop off with the loop on no patch cable (the level still effects the volume) it's pretty transparent. Adding a patch cable does increase the volume a small amount.

With the loop on and the loop level at max, with or without a cable, sounds more open and less congested. Granted it is a bit louder than with the loop off but raising the channel master to compensate only makes the congested sound louder.

From what I see the older SLOs didn't have a defeatable or variable level effects loop?

It seems that adding these options have this effect?

When I received the amp I always had an EQ in the loop so I didn't notice this.

Can’t speak from experience but lots of people always talk about the original SLO circuits FX loop affecting the sound considerably. Some people custom ordered versions without a loop and I think people get quite opinionated on which one they prefer. The V2 SLO’s moved the fx loop to after the tone stack, the V1 had the fx loop before (and was line level only)

 

[Elf]

So, when matching the level of the loop off with the loop on no patch cable (the level still effects the volume) it's pretty transparent. Adding a patch cable does increase the volume a small amount.

With the loop on and the loop level at max, with or without a cable, sounds more open and less congested. Granted it is a bit louder than with the loop off but raising the channel master to compensate only makes the congested sound louder.

From what I see the older SLOs didn't have a defeatable or variable level effects loop?

It seems that adding these options have this effect?

When I received the amp I always had an EQ in the loop so I didn't notice this.

In this video Dave Friedman interviews Mike Soldano and they talk about why the original design for the SLO100 ex loop, very interesting.

Ep. 19 - Mike Soldano of Soldano Custom Amps joins Dave Friedman and Marc Huzansky

Click here to purchase gear at Sweetwater.comhttp://bit.ly/2P1zPvDEp. 19 - Mike Soldano of Soldano Custom Amps joins Dave Friedman and Marc Huzansky

www.youtube.com

 

[Deadpan]

Can’t speak from experience but lots of people always talk about the original SLO circuits FX loop affecting the sound considerably. Some people custom ordered versions without a loop and I think people get quite opinionated on which one they prefer. The V2 SLO’s moved the fx loop to after the tone stack, the V1 had the fx loop before (and was line level only)

Seems the new loop is a completely different design.

I get the feeling that turning the loop level up is increasing the negative feedback but I have no idea how that would be possible.

 

[MirrorProfiles]

Seems the new loop is a completely different design.

I get the feeling that turning the loop level up is increasing the negative feedback but I have no idea how that would be possible.

it’s for the original soldano circuit but this explains some of why the fx loop affects the rest of the amp. It’s a good read in general about how the SLO works:

Soldano

How the Soldano SLO-100 & 50W Functions

robrobinette.com

 

[Deadpan]

it’s for the original soldano circuit but this explains some of why the fx loop affects the rest of the amp. It’s a good read in general about how the SLO works:

Soldano

How the Soldano SLO-100 & 50W Functions

robrobinette.com

Definitely interesting.

I wonder how the new and old are different?

Here is the description from the new on their website:

VERSATILE & TRANSPARENT​

Effects Loop​

The newly designed ultra-transparent tube-buffered effects loop handles anything from vintage pedals to high-powered rack units. For tube purists who prefer straightforward amp tone, a hardwire bypass switch is included allowing you to completely remove the effects loop from the circuitry. The new return level control can be used to set up two foot-switchable, preset volume levels.

 

[MirrorProfiles]

Definitely interesting.

I wonder how the new and old are different?

Here is the description from the new on their website:

VERSATILE & TRANSPARENT​

Effects Loop​

The newly designed ultra-transparent tube-buffered effects loop handles anything from vintage pedals to high-powered rack units. For tube purists who prefer straightforward amp tone, a hardwire bypass switch is included allowing you to completely remove the effects loop from the circuitry. The new return level control can be used to set up two foot-switchable, preset volume levels.

I’m sure as well as the loop being a better design, the fact they moved it to after the EQ means it’s way more useful and consistent with other amps.

As far as the interaction of the cathode follower of the old design’s fx loop vs the new one…. I have no idea. Maybe @James Freeman or @santiall or @Burger or @FractalAudio can provide some insight into it. I’m quite interested in what effect it has on the circuit on both implementations vs not having an fx loop

 

[santiall]

I’m sure as well as the loop being a better design, the fact they moved it to after the EQ means it’s way more useful and consistent with other amps.

As far as the interaction of the cathode follower of the old design’s fx loop vs the new one…. I have no idea. Maybe @James Freeman or @santiall or @Burger or @FractalAudio can provide some insight into it. I’m quite interested in what effect it has on the circuit on both implementations vs not having an fx loop

Hi, I don't know in the particular case of an SLO, or how different the old and new amps are but, in general, a cathode follower in a tube amp, when fed with relatively high voltages it'll clip and "compress" in a particular way.
The old SLO had two cathode followers being fed by high levels so I'd expect both to clip although I don't have an SLO to verify that. Perhaps that's part of the issues being discussed?

 

[headcrash]

Just for clarification:
The old SLOs used the following topology:
Preamp gain stages (3 for Lead, 2 for Normal)
Gainstage + DC coupled cathode follower into FX send (send buffer)
Gainstage + DC coupled chathode follower into tone stack (return recovery stage + tone stack "buffer")
Tone Stack
Master Volumes
Phase Inverter
Power Stage
-> FX loop including its two tubes is part of the signal path.

The new SLO supposedly has the following topology (at least accoridung to the fine folks at SLO clone forum, the schematic posted there may or may not have errors):
Preamp gain stages (3 for Lead, 2 for Normal)
Gainstage + DC coupled chathode follower into tone stack (tone stack "buffer")
Tone Stack
Master Volumes
AC coupled cathode follower into FX send (send buffer)
Gainstage (return recovery)
FX volume
Phase Inverter
Power Stage
-> FX loop including FX volume pot is switched in or out

My concern with designing a loop like this, and this is particularly true when you would want to use the loop's "footswitch-ability", the signal levels around the loop are highly dependent from where you set your master volumes.
Like I described above, the master volumes are essentially your send level pots, when the loop is engaged, and the FX level pot at the back of the amp acts as your actual master volume.

What's perplexing is, that Deadpan hears a small volume increase when adding a patch cable and loop engaged. Because with or without cable the signal path should be literally the same, and so should volume.

I think, without modding the amp, you might need to get along with the behaviour, assuming everything is working properly with that loop. I mean, an ever so small tone alteration with the loop on, I'd say, can be expected, even without driving them into saturation, since we have two tube stages added in and out of the signal path. This wasn't the case with the old SLO.
If you experience your best sound with loop level at max, I assume, you would use the channel master volumes to control you actual playing volume. I'd expect you won't be able to turn up the masters so high that the FX unit or the two tube stages would really distort, without severely hurting yourself.

 

[Deadpan]

Yep, with the loop level at max I definitely have to keep the master lower.

I haven't noticed another amp behave like this so it was unexpected.

Time to see if any of my other amps have this effect with the loop.

 

[Deadpan]

Here is a test clip to show the difference.

1st is loop on max, second is loop off and third is loop off and the master turned up to try to match the volume of the of the first clip.

This is not and ideal sound but rather the gain set to 5 on the overdrive channel.

When I set this tone up, I was boosting it. For the sake of comparison I turned the boost off and lowered the volume some.

 

[headcrash]

Hey thanks. This helps a lot!
I assume there is zero processing in the recording, no normalising or whatsoever.
To my ears the first bit of the clip sounds considerably better, more clarity and openness, whereas the third is a bit dull and quite swampy.
Where is the lead master set for the third bit?
Where is it set for the first bit?
Am I getting you right, that the lead master is set the same for the first and second bit?

 

[Deadpan]

Correct, no processing or normalization.

The master was set to 1 for clip one and two. Had to raise it to around 1.5 for the third clip to be a similar level to clip 1.

 

[headcrash]

Ok, neither the 1 of 11, nor the 1.5 of 11 setting should be high enough to drive the tube stages of the loop into serious saturation (which is what the sound of the third bit could be interpreted as). But keep in mind, this is my gut feeling, and we'd only know for sure, until we would probe this with a signal generator and scope.
Maybe others can chime in and share their opinions.

Still I wonder, if there is something wrong with the loop. Just for the fun of it, have you tried changing the FX loop tube? It should be the one in the back, next to the power tubes.
If the tube isn't faulty, a tech should check and/or compare the wiring and the values of the components of the loop. I doubt that B.A.D. uses automated population of their pcb's, so I see the possibility of one or more falsely populated components. During final check, such a mistake may or may not be noticed. On first sight, the loop works as it should, but it does not so properly.

 

[Deadpan]

I did change the fx loop tube a few days ago to see.

Note this difference in sound is present at all volumes. I kept the master low just to rule out power amp distortion for the clip.

Honestly, I'm ok with it as I know to just leave the loop on and up.

It would be interesting to know if others have the amp and experience the same.

 

[headcrash]

The AC coupled cathode follower before the send has almost unity gain. Then there is a ~12dB voltage divider (15k/4.7k) before the send jack (at least according to that schematic on Sloclone forum).
The recovery stage there has a gain of roughly 50 which corresponds to over 30 dB (220k plate, 1.5k cathode, someone correct me please if I am wrong).
To me, this all makes sense, if you would have to set your master volumes quite low, for the use of pedal level FX units. You will need quite an amount of gain to make up for the attenuation and to achieve similar level values like without the loop.
If you'd use some studio grade line level FX device, you might be able to turn up the master volumes quite a bit more, and compensate by turning the loop level down a bit.
Even if you want to just use the loop as a volume boost, you would use it in a similar fashion. Just without FX unit.
All this should be able to be done without such a drastic change in sound. At least that would be what I'd expect from a 4000 USD amp.

Would indeed be interesting to hear from other SLO Mk 2 users how their's reacts.