Reamper/Pedal-Interface with filters, input needed

[Studio Mollan]

Hi,
I started drawing on this a while back. It's a 500 series module that converts +4 balanced signals to unbalanced via transformer, then high impedance unbalanced to +4 balanced again. Much like the Radial EXTC500. I already own one of those and it's great! Makes me able to use my Space Echo as a +4 insert with a minimum of noise and great sound.
I plan to use it as either a loop for old gear that actually needs a 1Mohm input inpedance, (like the '63 fender reverb) and also as a normal +4 to -10 converter. The way I see it it should work for both? or? Any opinions greatly appreciated. EVERYTHING inserted here will be of some sort of LOFI nature so it does not have to have 20-20k bandwidth and other features considered hifi.

I have a spare 500-chassie that I'm planning to fill up with these. It's not really worth a lot of money anyways and if I succeed with this design it will become a cost effective alternative to the Radial. If I get it working I will offer PCBs to anyone interested.
One addition to the design is that it also incorporates a high,- and low-pass filter, I went with a design I've already used in diy, the Harrison Ford filter.

I would appreciate any feedback on the design. I'm not too comfortable drawing up my own designs. Iv'e looked at other designs, borrowed and modified them to suit (or hopefully suit that is).

Signal Flow:
Input is a NE5532. One half as a balanced receiver and the other as a variable send gain. Then a transformer, via a polarity switch to a front panel mounted 1/4" jack.
Signal returns via another front panel mounted 1/4" jack that feeds a OPA604 high impedance buffer. From the buffer into the filter and filter switch.
The filter Switch feeds the output amp either signal straight from the buffer or from the filter output. The signal is always present in the filter, the switch just switches the filter output to the output amp. The output amp is another NE5532 wired as a balanced line driver with variable gain.
I'm a bit ashamed of the output section.. It's pretty much the output section of the JLM audio Dingo PUP....
I might just replace the output amp with a THAT1646. A bit more expensive but worth if if not intruding on others designs. If I use the THAT1646 I will ned to reconfigure the OPA604 with variable gain. Using I as a standard non-inverting op amp with gain instead of a buffer. Probably fine.

The frontpanel will have: 4 pots; HighPass, LowPass, Send and Return. 2 Switches; Polarity and Filter. 2 LEDS for Polarity and filter. And finally 2 1/4" jacks for interfacing the pedal/tapedelay/etc.

I'm not sure about the transformer. Radial uses it in the EXTC500 and that sounds pretty sweet. The zobel network is from the Radial box. That I might have to recalculate at a later stage, when I have a working prototype. I'm not sure I really need it, or if it's going to work. Opinions here if you have any.

So, if you see any errors or such please comment. I think I've taken it as far as I can.





 

 

[Studio Mollan]

Here's the schematics for the filter.

 

[abbey road d enfer]

Input is a NE5532. One half as a balanced receiver and the other as a variable send gain. Then a transformer, via a polarity switch to a front panel mounted 1/4" jack.

You need to check polarity. As I see it, the normal position is reverse-polarity. Not a big deal, since you have an inverter.

I'm a bit ashamed of the output section.. It's pretty much the output section of the JLM audio Dingo PUP....

And it's flawed. It is the most embarassing topology for a balanced output, since it does not tolerate being connected to unbalanced inputs, or loses 6dB if used as an unbalanced output.
I belive there are two essential alternatives: either a cross-coupled electronically balanced circuit, for which I would favour a THAT1646 over a "discrete two opamp implmentation, or a simple impedance-balanced output, just one opamp and a RC circuit at pin3.

I'm not sure about the transformer. Radial uses it in the EXTC500 and that sounds pretty sweet.

I'm not a fan of transformers, probably because I know too much about them. 
But in this particular case, it would make sure you don't run into ground loop issues.

 

[Studio Mollan]

Thank you!!

You need to check polarity. As I see it, the normal position is reverse-polarity. Not a big deal, since you have an inverter.

I saw the polarity issue now- You are right. The second stage of the input is inverting so the first should be swapped.

And it's flawed. It is the most embarassing topology for a balanced output, since it does not tolerate being connected to unbalanced inputs, or loses 6dB if used as an unbalanced output.
I belive there are two essential alternatives: either a cross-coupled electronically balanced circuit, for which I would favour a THAT1646 over a "discrete two opamp implmentation, or a simple impedance-balanced output, just one opamp and a RC circuit at pin3.

That settles it then. That1646 or impedance balancing for the output.
The impedance-balanced option is tempting as I could replace the OPA604 with a OPA2604. Use half as the buffer and the other half for output. The "no variable gain"-issue with THAT1604 would also be solved.
Great idea! Thank you!

I'll redraw and post again in a few hours

/
Emil

You need to check polarity. As I see it, the normal position is reverse-polarity. Not a big deal, since you have an inverter.
And it's flawed. It is the most embarassing topology for a balanced output, since it does not tolerate being connected to unbalanced inputs, or loses 6dB if used as an unbalanced output.
I belive there are two essential alternatives: either a cross-coupled electronically balanced circuit, for which I would favour a THAT1646 over a "discrete two opamp implmentation, or a simple impedance-balanced output, just one opamp and a RC circuit at pin3.
I'm not a fan of transformers, probably because I know too much about them. 
But in this particular case, it would make sure you don't run into ground loop issues.

 

[john12ax7]

Since you have a transformer why even bother with the opamp input? The transformer itself is a great way to do balanced to unbalanced.

 

[Studio Mollan]

A good point. I went with an active input because of the variable gain, and cost.
If I went with a transformer input I would want a "good" transformer and that would add to the cost.
Also an active input will perform the same regardless of what you feed the module with.
This is a "gut feeling" approach and not based on clear facts, so I might be wrong.
The transformer option would need a 10:1-ish transformer?

Im actually leaning towards removing the transformer. When I get a prototype up and running I'll see pretty quick if I gain anything from it or not. If not, I will go with the active input alone.
/
Thanks for your input!
/
Emil

Since you have a transformer why even bother with the opamp input? The transformer itself is a great way to do balanced to unbalanced.

 

[abbey road d enfer]

Since you have a transformer why even bother with the opamp input? The transformer itself is a great way to do balanced to unbalanced.

The xfmr is in the output to the guitar amp, for isolation. The input stage needs to be balanced in order to accept connections from balanced lines.

 

[john12ax7]

The xfmr is in the output to the guitar amp, for isolation. The input stage needs to be balanced in order to accept connections from balanced lines.

A lone inline transformer would accomplish both of those things.

 

[Studio Mollan]

Alright, I have redrawn the schematic now.
I'm not at all sure about the new output amp. I just found a few examples online and modified to fit my needs. A 10k input impedance with 20k gan pot. Should give a decent gain range right?

Would it be worth to scrap the impedance balancing and instead put a THAT1646 after the output amp?
Theres room on the board for sure and it's not adding much to the cost of the project. I'm not building a precision instrument here but I don't want it to feel limited. Rather spend a little more money one it in that case

I marked a few spots in the schematics where I'm on thin ice. Do you always want a capacitor on the output of OP-amps and if so; why, and how do you calculate the values? Also the resistors on the output, they might not be needed?

 

[Studio Mollan]

Just remembered this. Read if a couple of years ago. Would this be applicable for my design?
Convert the second stage of the input amp to this and drive the transformer according to this circuit?
Might be overkill? Or not?
https://www.lundahltransformers.com/wp-content/uploads/datasheets/feedbck.pdf

/
Emil

 

[abbey road d enfer]

Would this be applicable for my design?

This is an excellent option, but it just improves the LF response.
You must be aware that the values must be tuned to the specific xfmr.

 

[RSRecords]

This is an excellent option, but it just improves the LF response.
You must be aware that the values must be tuned to the specific xfmr.

I would say unnecessary for the goal of the project, but to be fair it doesn't complicate the build so much-- just the design. You might want less LF response going into pedals. 

This is a cool project, by the way. I'd definitely be in for a pair of pcbs when it's ready!

 

[Studio Mollan]

This is an excellent option, but it just improves the LF response.
You must be aware that the values must be tuned to the specific xfmr.

the more I read up on it the more I think it would suit my design.

The Allied transformer I have in my design appears to only be available in bulk, straight from allied. Haven't asked them how many you need to buy but it might be FAR to many for my needs.
so I started looking at the Xicom equivalent.
That is claimed to be within +-3dB from 300Hz to 3400kHz. Would it be safe to assume that it cuts off bass @ 300Hz and that it starts to increase HF @3400kHz.
In that case it could be totally useable with a properly tuned zobel network to tame the hi-end, and mix feedback drive to extend the low end.
Am I right? right-ish?

The Allied is specified as a "High frequency transformer". The only information on bandwidth that is specified sadly.

 

[Studio Mollan]

Hi!
indeed, I don't feel I need 20Hz low end, but 300H is a bit to steep for me...
IF I get this done I will definitely offer some PCB:s. I have lots to do now so probably not in a couple of months.

I would say unnecessary for the goal of the project, but to be fair it doesn't complicate the build so much-- just the design. You might want less LF response going into pedals. 

This is a cool project, by the way. I'd definitely be in for a pair of pcbs when it's ready!

 

[Studio Mollan]

This is starting to look good.
Im guessing this has a better f-response straight ot of the box.
https://www.don-audio.com/Edcor-PC10K-10K

 

[Studio Mollan]

Oh, and i forgot. I got my batch of prototype cards last week.
Looks good but made a few errors. Lots of the pads are either too large or to small. The switches were hard to solder to position due to too large holes, and the Neutrick jacks won't fit in the hoes as they are too small. The caveat of working with public components in easyEDA.

Other than that it looks good. Waiting for a batch of components now so should be able to fire it up pretty soon.
I did a front plate made of PCB as well. Came out WAY above expectation! If this works without the screening form a metal enclosure this is a viable solution. Would be great not having to order metal parts....

 

[RSRecords]

Oh, and i forgot. I got my batch of prototype cards last week.
Looks good but made a few errors. Lots of the pads are either too large or to small. The switches were hard to solder to position due to too large holes, and the Neutrick jacks won't fit in the hoes as they are too small. The caveat of working with public components in easyEDA.

Other than that it looks good. Waiting for a batch of components now so should be able to fire it up pretty soon.
I did a front plate made of PCB as well. Came out WAY above expectation! If this works without the screening form a metal enclosure this is a viable solution. Would be great not having to order metal parts....

Looks great!

 

[Studio Mollan]

Thanks!
Just got the transformers today. There rests of the components should arrive soon.
/
Emil

Looks great!

 

[Meathands]

Cool project! Regarding the output amp, check DC offset once you've built it up. Since you have gain in the output amp you may want a coupling cap on the output.

And the impedance balanced output seems needlessly complex. All you need is something like what's attached.

 

[Studio Mollan]

Thanks!
I’ve changed the circuit a bit since the last upload. I decided to go with a that1646 as output.

The latest version is attached here. If you have the time a short look through would be great!!

Also if you have any input on my last post, the input protection circuit with a pair of zeners. Still not sure if I feel I need them, but might be an idea to make this as “fool proof” as possible.

Right now I’m waiting for components. As soon as they get here I’ll fire one prototype up and see how it works. Pretty sure it will work fine after a little tweaking, my biggest concern now is wether the transformer will be needing more shielding or not. Guess I just have to build one and see/hear how it performs.

Cool project! Regarding the output amp, check DC offset once you've built it up. Since you have gain in the output amp you may want a coupling cap on the output.

And the impedance balanced output seems needlessly complex. All you need is something like what's attached.