A tube tracking console using ITB’s MkIII concepts

[rjuly]

Yes, that makes sense. However, it might be a problem for me, in that if I place the insert post transformer, then I won't be able to monitor the effect of the insert, since this feed to the headphones would presumably be taken before that output transformer? Or would my monitor feed also be a balanced post tranformer feed?

- Richard

 

[ruffrecords]

Yes, that makes sense. However, it might be a problem for me, in that if I place the insert post transformer, then I won't be able to monitor the effect of the insert, since this feed to the headphones would presumably be taken before that output transformer? Or would my monitor feed also be a balanced post tranformer feed?

- Richard

What I described was the way we did it at Neve back in the 70s. And the way we monitored then was at the output XLR via a (31267) 10K:600 trasnformer into the monitor section (which is basically what you have in your original block diagram).

Cheers

Ian

 

[rjuly]

What I described was the way we did it at Neve back in the 70s. And the way we monitored then was at the output XLR via a (31267) 10K:600 trasnformer into the monitor section (which is basically what you have in your original block diagram).

Cheers

Ian

Yes, I had assumed the headphone circuit would be fed with an unbalanced connection. I am planning to use your 'improved' headphone circuit design - I take it I can feed that with a balanced input. The fun bit will be finding space for those %&*^$ lovely Sowter output transformers! ;-)

BTW - what would be the ideal potentiometer value to feed your headphone amp design?

… Come to think of it, for a balanced volume control I would need a 4 gang pot, correct?

- Richard

 

[ruffrecords]

The headphone amp has an unbalanced input. If you want to connect it to a balanced signal then you really need to unbalance the signal which usually means an input transformer. On the very first mixer I built I used a very simplified version of the headphones amp - just the ECC99 SRPP output stage on its own with no NFB. At the input I had a couple of OEP Z3003E input transformers which only cost £13 each. Once the signal is unbalanced you can feed it into a regular dual gang 10K volume pot on its way to the phones amp.

Cheers

Ian

 

[rjuly]

Looking at the design I can see - of course - it's unbalanced (doh!). Also, I can see see there is no way around it - signal has to be balanced for inserts and line out, and unbalanced to enter another tube circuit. What's another couple of transformers, eh?

In fact, I will be relying on the headphone amp both while recording and mixing acoustic live music, both contemporary and classical, so I need to make the headphone amp as pristine as possible. I am happy to put some good quality low distortion transformers on the front end, and go with your full design with NFB. I do think I may have to increase the chassis size to accomodate it.

A friend of mine is willing to give me a good price on a bunch of OEP transformers (10K to 10K Line Input 1+1:1 A187A13C ), which I think would work well for my insert returns and line outputs. Would it not also work as a line bridge into the HP circuit? Thanks for your efforts leading the lame and halt ;-)

- Richard

 

[rjuly]

As my design (version of Ian's..) stands now, I have two spare backplane slots which I would like to use to incorporate passive EQ and possibly compression that could be physically patched into the inserts of either individual channels, or across the stereo bus. Those circuits would only use power and io from the backplane and not participate on the main bus. My question is whether it is feasible to cram a pair of eqs onto a single 35mm channel and likewise two channels of compression onto the other. If this seems unlikely (crazy), then perhaps I might be better to forego the compression and simply build out the best eq I can, one per channel.

- Richard

 

[rjuly]

An updated block diagram...



- Richard

 

[ruffrecords]

A friend of mine is willing to give me a good price on a bunch of OEP transformers (10K to 10K Line Input 1+1:1 A187A13C ), which I think would work well for my insert returns and line outputs. Would it not also work as a line bridge into the HP circuit? Thanks for your efforts leading the lame and halt ;-)

- Richard

These will be fine as line level input transformers but they are not suitable as output transformers. If you want a compact 2K4:600 output transformer at a reasonable price, OEP do the K30A11C. This is not available through distribution or shown on their web but it is a standard product. They do a special version for me with an iron core which will handle more level at 20Hz.

Cheers

ian

 

[ruffrecords]

A couple of points on the latest version of the block diagram. First the 2:1 output transformer has a loss of 6dB so it should be shown as -6dB not 0dB.

I am not quite sure how your solo/pfl is going to work. You seem yo be taking the solo signal from a balanced output to a bus and then straight to the monitor without a bus amp. The usual (Neve) way to do solo/pfl is to take an unbalanced signal pre-fader to bus, then via a bus amp and output transformer to the monitor relay.

Cheers

Ian

 

[rjuly]

A couple of points on the latest version of the block diagram. First the 2:1 output transformer has a loss of 6dB so it should be shown as -6dB not 0dB.

Thanks for spotting that - I will fix that.

I am not quite sure how your solo/pfl is going to work. You seem yo be taking the solo signal from a balanced output to a bus and then straight to the monitor without a bus amp. The usual (Neve) way to do solo/pfl is to take an unbalanced signal pre-fader to bus, then via a bus amp and output transformer to the monitor relay.

I guess that makes two of us. My thinking is that I need to take the PFL feed from post insert (to ensure I can hear what the insert is doing ) - which is balanced at that pont, and then bring it back post main outputs (to ensure that the PFL only goes to monitoring and leaves the main outputs alone) - which is also balanced. To be honest I hadn't really thought through the PFL, but it is obvious that it is a bus, and therefore would need a bus amp amp and transformer. I suppose it would need to be a balanced bus, with dual bus amps and transformers? Is this a viable concept, or do I need to completely rethink how PFL is done?

- Richard

 

[rjuly]

These will be fine as line level input transformers but they are not suitable as output transformers. If you want a compact 2K4:600 output transformer at a reasonable price, OEP do the K30A11C. This is not available through distribution or shown on their web but it is a standard product. They do a special version for me with an iron core which will handle more level at 20Hz.

Cheers

ian

I had thought to standardize using the Electro-Mag E1257 as my output transformer. I have already ordered a tranche of them (let me know if you ever need a couple on short notice). These would be used for the channel outputs feeding the inserts, and the transformer that feeds the main outputs from the 2 bus - although I see from your previous post that that the bus output should be 10K:600.
I had thought to use the 10K:10K OEMs as my insert returns, and the inputs for the outboard modules (EQ and compression) and to unbalance the input to the headphone amp. Is that not the correct transformer to use for feeding the HP amp - should it also be a 2K4:600 output transformer?

- Richard

 

[rjuly]

After scratching my head a bit, here's a new block diagram incorporating an unbalanced PFL circuit...

 

[ruffrecords]

I had thought to standardize using the Electro-Mag E1257 as my output transformer. I have already ordered a tranche of them (let me know if you ever need a couple on short notice). These would be used for the channel outputs feeding the inserts, and the transformer that feeds the main outputs from the 2 bus - although I see from your previous post that that the bus output should be 10K:600.
I had thought to use the 10K:10K OEMs as my insert return, and the inputs for the outboard modules (EQ and compression) and to unbalance the input to the headphone amp. Is that not the correct transformer to use for feeding the HP amp - should it also be a 2K4:600 output transformer?

- Richard

I forgot you are using the ElectroMag output transformers. They are 2K4:600 which is exactly what you need to balance the output from the twin line amp. 10K:10K is just what you need for any line level input like insert returns.

The only place you might use a 10K:600 transformer is in the monitor section but the way you have arranged the switching avoids this. I will draw a sketch of how to do the PFL.

Cheers

Ian

 

[rjuly]

I am going to have to write a laundry list for my transformers - after some financial peregrinations I have decided to plump for Sowters for all transformers besides the Electro-Mags, and I will need to place an order - the lead times are long, as you well know. Since I am a Yank, I would normally go for Jensens, but Brexit has put paid to that. Buy British!

- Richard

*edit - I make it:

Channel output 8 ch__8x__Electro-Mag E1257
Bus Amp/PFL out____3x__Electro-Mag E1257
Eq out 2 ch__________2x__Electro-Mag E1257
Channel input________8x__Sowter 9610 200/20k Microphone input (PCB)
HP unbalancing______2x__Sowter 3575 10k/10kCT Isolator-Balancer (PCB)
Insert returns________8x__Sowter 3575 10k/10kCT Isolator-Balancer (PCB)
Eq in 2 ch___________2x__Sowter 3575 10k/10kCT Isolator-Balancer (PCB)

Total:

13x__Electro-Mag E1257
12x__Sowter 3575 10k/10kCT Isolator-Balancer (PCB)
08x__Sowter 9610 200/20k Microphone input (PCB)

Yowza!

 

[ruffrecords]

20 x Sowter transformers are going to cost you around £1600. You are probably right now going through the process I went through a few years back - wondering how to reduce the cost of transformers. You can probably halve that by using OEP alternatives (and they are still British).

Cheers

Ian

 

[rjuly]

20 x Sowter transformers are going to cost you around £1600. You are probably right now going through the process I went through a few years back - wondering how to reduce the cost of transformers. You can probably halve that by using OEP alternatives (and they are still British).

Cheers

Ian

You have built quite a few consoles with, I imagine, most of the possible options, so it's likely you know whereof you speak ;-) Is there no discernable difference between the OEP alternatives and the top shelf tranformers, or is it more a matter of accepting 95% (or whatever the relative values) of the performance for 50% of the cost? Perhaps another way of putting it is: If price were no object, would you still be happy with the OEP alternatives for your console?
- Richard

 

[ruffrecords]

You have built quite a few consoles with, I imagine, most of the possible options, so it's likely you know whereof you speak ;-) Is there no discernable difference between the OEP alternatives and the top shelf transformers, or is it more a matter of accepting 95% (or whatever the relative values) of the performance for 50% of the cost? Perhaps another way of putting it is: If price were no object, would you still be happy with the OEP alternatives for your console?
- Richard

Interesting question. I have only once measured a Sowter transformer, you usually just take it on faith that they will be good. The one I tested was their 1461 which is 2K4:600 output transformer. It was equal to or better than the OEP and Carnhill at mid frequencies but below 40Hz its distortion rose unacceptably quickly compared to the others. Overall I judged its performance to be worse than the other two. I had the OEP one modified so its performance equalled or bettered the Carnhill and it is still only £40. Of course, for a compact output transformer the ElectroMag is the best.

I tested lots of OEP input transformers, both 10K:10K types and aa 1:10 mic input ype. Of thee 10K:10K types, the A187A13C was quite poor, the tiny Z21807C was pretty good despite only costing £12, and the £40 K30A06C was exceptionally good turning in a performance I would expect from a Sowter. Equally the 1:10 A187A15C mic input transformer measured far better than I expected and again turned in a performance as good as any Sowter, Cinemag or Jensen.

As to what they sound like only tour ears can tell you. My ears are 70 years old so they are very experience but a little worn. I cannot hear any difference between the best OEPs and any Sowter.

Cheers

Ian

 

[rjuly]

Interesting question. I have only once measured a Sowter transformer, you usually just take it on faith that they will be good. The one I tested was their 1461 which is 2K4:600 output transformer. It was equal to or better than the OEP and Carnhill at mid frequencies but below 40Hz its distortion rose unacceptably quickly compared to the others. Overall I judged its performance to be worse than the other two. I had the OEP one modified so its performance equalled or bettered the Carnhill and it is still only £40. Of course, for a compact output transformer the ElectroMag is the best.

I tested lots of OEP input transformers, both 10K:10K types and aa 1:10 mic input ype. Of thee 10K:10K types, the A187A13C was quite poor, the tiny Z21807C was pretty good despite only costing £12, and the £40 K30A06C was exceptionally good turning in a performance I would expect from a Sowter. Equally the 1:10 A187A15C mic input transformer measured far better than I expected and again turned in a performance as good as any Sowter, Cinemag or Jensen.

As to what they sound like only tour ears can tell you. My ears are 70 years old so they are very experience but a little worn. I cannot hear any difference between the best OEPs and any Sowter.

Cheers

Ian

Hi Ian,
Thank you for laying that out so succinctly. What you say is very compelling and I believe I will go with your recommendations of the K30A06C and the A187A15C. The great attractions of price and availability don’t hurt either ;-)
Ears my be a bit worn (I know mine are at 58) but the brain is amazing at analysing and and interpreting what gets through, especially in relative terms. I should think your impressions are accurate. I happen to have a sowter 9610 on order for another project (when it finally arrives...), and it should be interesting to compare it with the K30A06C.
- Richard

 

[rjuly]

Output transformers have arrived - had to order the minimum - that's a lot of iron...



Looks nice though.

- Richard

 

[ruffrecords]

happen to have a sowter 9610 on order for another project (when it finally arrives...), and it should be interesting to compare it with the K30A06C.
- Richard

Note that the 9610 is a 1:10 mic input transformer and the K30A06C is a 10K:10K line level input transformer so not quite apples and apples. I think you may have meant to compare the 9610 with the A187A15C.

I see you got the lovely big ElectroMag output transformers. The have a lot of windings, Do you have the pin out and how to wire them up info?

Cheers

Ian