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Adding transformers to consumer gear
Old 26th July 2020
  #1
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Adding transformers to consumer gear

I have an unbalanced piece of hi-fi gear that I would like to use in the studio. Could I add transformer inputs and outputs to make it better interact with my studio gear? If so, what ratio would be best?
Old 26th July 2020
  #2
Depends on the equipment you want to interface. 10K to 600 ohms on the input and <it depends on the output impedance of the consumer gear> to 600 Ohms on the output. Some consumer equipment will not be able to drive a transformer to full output. Suggest something like this instead. https://reverb.com/item/34013658-aph...SABEgJPJfD_BwE All in one unit.

Good discussion here https://crookwood.com/blog/dealing-w...in-the-studio/

Best of luck!
Old 26th July 2020
  #3
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I think that while looking at transformer ratios in terms of impedance is traditional, it's a very confusing way to think of it in this application. Here's what I consider to be the main points:

First, use input transformers on equipment inputs, and output transformers on . . . you guessed it . . . equipment outputs. Input transformers are designed with Faraday shields for better isolation, and they're tolerant of a wide range of source impedances . . . but they need to have a very short amount of cable between them and the equipment (low capacitance), and need to be loaded properly on their output side for best performance. Output transformers are the opposite -- to work their best, they need a low driving impedance from the electronics, and can be more tolerant of different loads and cable lengths on their output . . . this requires tighter coupling across the windings and no Faraday shield, so they deliver less isolation.

For the ratio, you'll generally want 1:1 for line-level inputs and outputs . . . unless you're in the specific situation where the input side of of the consumer gear can't handle the generally higher signal level that professional equipment puts out . . . in which case a 4:1 input transformer works well, as it knocks down the signal by -12dB. This is the crux of the matter in the whole "+4 vs. -10" discussion . . . if it's something like a consumer tape deck where you can simply crank down the input pot and all is well, there's nothing to worry about. If it's i.e. a consumer equalizer that has limited headroom and no input adjustment, then you'll have to do something to prevent overload. For the output side, you'll need to be able to crank up the gain of the following piece of gear to compensate for the level difference . . . if this isn't possible, then attempting to make up the gain with i.e. a 1:4 output transformer will usually have significant sonic side-effects.
Old 26th July 2020
  #4
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Thanks for the replies! Some more information about the piece: 75K input impedamce, and 10ohm output. I'm pretty sure it's made for -10 cHi level. It does not have input or output level controls.
Old 27th July 2020
  #5
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I'd suggest Jensen 10KB-D for an input, 4:1 ratio . . . an extremely neutral, clean-sounding transformer that I've used in several designs. Cinemag CMLI-10/600 is also very good. Cinemag CMOB-2S and Jensen 11-EM would be a couple of good choices on output, both 1:1 ratio. Lundahl 5402 will also work and give you the option of wiring as a 1:2 . . . a low 10 ohm output impedance is pretty uncommon on consumer gear, and the step-up configuration can take advantage of this and give you back half of the voltage that you lost on the input side. Anecdotally, I've found the Lundahl iron to be a bit more sensitive to loading . . . but this may or not be the case with this specific part in your application, so take that with a grain of salt.

If you have any concern about the unit's noise floor, you might set up a test with your DAW to see exactly how much headroom it has. +/-12-volt power-supply rails are common in consumer gear, which will usually allow clean signal levels up to +18dBu with opamp-based designs . . . but it warrants testing to be sure, and do your test with controls in various positions to check for any internal gain-structure bottlenecks. If you find that it has decent headroom, then using a 1:1 input transformer will give you a lower total noise floor, and more consistent levels across your patchbay.
Old 27th July 2020
  #6
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Quote:
Originally Posted by kirkus View Post
I'd suggest Jensen 10KB-D for an input, 4:1 ratio . . . an extremely neutral, clean-sounding transformer that I've used in several designs. Cinemag CMLI-10/600 is also very good. Cinemag CMOB-2S and Jensen 11-EM would be a couple of good choices on output, both 1:1 ratio. Lundahl 5402 will also work and give you the option of wiring as a 1:2 . . . a low 10 ohm output impedance is pretty uncommon on consumer gear, and the step-up configuration can take advantage of this and give you back half of the voltage that you lost on the input side. Anecdotally, I've found the Lundahl iron to be a bit more sensitive to loading . . . but this may or not be the case with this specific part in your application, so take that with a grain of salt.

If you have any concern about the unit's noise floor, you might set up a test with your DAW to see exactly how much headroom it has. +/-12-volt power-supply rails are common in consumer gear, which will usually allow clean signal levels up to +18dBu with opamp-based designs . . . but it warrants testing to be sure, and do your test with controls in various positions to check for any internal gain-structure bottlenecks. If you find that it has decent headroom, then using a 1:1 input transformer will give you a lower total noise floor, and more consistent levels across your patchbay.
Thanks for.your reply. Some.good info there. This is unit is discrete, and the transistors are.getting +43V according to the schematic.
Old 29th July 2020
  #7
What Unit

Jumping in out of curiosity as to what unit this is.

I also have a consumer level EQ that is fully discrete and inductor based with 40+volt rails and was searching how to balance it with transformers.
Old 29th July 2020
  #8
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Quote:
Originally Posted by ShardsLV View Post
Jumping in out of curiosity as to what unit this is.

I also have a consumer level EQ that is fully discrete and inductor based with 40+volt rails and was searching how to balance it with transformers.
A Realistic 31-1987. It's a stereo 5 band graphic EQ.
Old 29th July 2020
  #9
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Originally Posted by craigmorris74 View Post
A Realistic 31-1987. It's a stereo 5 band graphic EQ.
I just saw one for sale locally. How is it? How does it sound stock?
Old 29th July 2020
  #10
HA! That's exactly what I have an am looking to balance!

I have a lot of solid gear, neve, capi/api, pultec, hairball 1176 and that goofy eq hangs with all the big kid stuff. I've even strapped it across final mixes.

Interested in hearing how your upgrades go--I'm just now opening my studio back up so it'll be a bit until I can resume justifying these kind of projects.

Cool to find another user of this eq out there!
Old 29th July 2020
  #11
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Quote:
Originally Posted by ShardsLV View Post
HA! That's exactly what I have an am looking to balance!

I have a lot of solid gear, neve, capi/api, pultec, hairball 1176 and that goofy eq hangs with all the big kid stuff. I've even strapped it across final mixes.

Interested in hearing how your upgrades go--I'm just now opening my studio back up so it'll be a bit until I can resume justifying these kind of projects.

Cool to find another user of this eq out there!
Mine is coming in the mail today. Glad to hear that it sounds good! The schematic looks promising!
Old 29th July 2020
  #12
Quote:
Originally Posted by enginefire View Post
I just saw one for sale locally. How is it? How does it sound stock?
I've kept it stock for years because it sounds solid enough to not bother with the transformers. It was usually kept at home where I'd do a final listen on mixes and would somtimes run some finals thru it before bringing it back to the studio to master.

In one case I even powered it off while capturing a song to give it a crunchy, lofi, circuit-starved abrupt fade out (it was experimental electronic music).

I don't want to bias your perception of it too much before you get to hear it so I'll just say that all the bands are useful depending on source, they don't sound bad cranked, and you can pump a lot of signal into it.

Definitely a cheap lil sleeper!
Old 29th July 2020
  #13
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Quote:
Originally Posted by ShardsLV View Post
I've kept it stock for years because it sounds solid enough to not bother with the transformers. It was usually kept at home where I'd do a final listen on mixes and would somtimes run some finals thru it before bringing it back to the studio to master.

In one case I even powered it off while capturing a song to give it a crunchy, lofi, circuit-starved abrupt fade out (it was experimental electronic music).

I don't want to bias your perception of it too much before you get to hear it so I'll just say that all the bands are useful depending on source, they don't sound bad cranked, and you can pump a lot of signal into it.

Definitely a cheap lil sleeper!
Awesome, I might go by tonight and pick it up!
I was looking at the schematic. I guess there is no easy way to adjust the frequencies. Like it’s inductor that sets the frequency. It would be nice to be able to add a switch to shift the bands a bit.
Old 31st July 2020
  #14
Yeah from what I remember of a Pultec schematic, they have a rotary switch between the different value inductors.

I have an old SoundCraftsmen stereo eq that's all inductor sitting around if you need a source for parts. I don't imagine it'd be too hard to toggle between two inductors per band.
Old 31st July 2020
  #15
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Originally Posted by ShardsLV View Post
Yeah from what I remember of a Pultec schematic, they have a rotary switch between the different value inductors.

I have an old SoundCraftsmen stereo eq that's all inductor sitting around if you need a source for parts. I don't imagine it'd be too hard to toggle between two inductors per band.
Thanks shards, that’s very generous of you. I haven’t opened it up to see what kind of room there is inside. Would it be possible to change the frequency by changing the cap in the band circuit?
Old 31st July 2020
  #16
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You could also try something like the Henry Engineering Matchbox which
is active but transformerless. Converts -10 unbalanced to +4 Balanced
and vice versa.
Old 31st July 2020
  #17
Yeah you could totally use an active balancing box too. I have a couple of those ATI ones you might find in a radio station at my studio and have tried them with success on old RCA jacked gear. I usually just use the Realistic EQ stock with a short cable run and I'm good so being able to balance it isn't a deal breaker.

I know for me more of the magic of balancing with transformers is that it somewhat (analog-romantically) brings it closer to the designs of big kid gear like API and Neve inductor EQs and such, and putting things thru transformers just sounds cool, and is especially rewarding when you DIY. A bunch of metal bass I tracked went thru a UREI trafo I had laying around and it really made it sit nicely and was fun to experiment with.

Dug thru a few scholarly articles about LC circuit design and after the calculus-induced nausea wore off, remembered a thread here on GS where someone touched on cap / inductor values. I'll quote it below as well as link to the og thread.

Also, I have like 18-20 DBX dolby NR cards, all of which are loaded with in and out trafo's, germanium transistors, and 2 black can discrete VCA's each. Thinking about offloading a bunch to intrepid DIY'ers.

Anyway, here's the quote and thread!

"In equalizers big inductors allow the use of small capaciators.. and as it looks..as smaller the involved caps and bigger the inductors.. as fatter the sound.. See the moog FilterBank for this..

In any case its easier to get high quality caps in low values.. while inductors are way more expensive than capaciators.. So moog made here not a economic decision.. Same filter bank could have been realized with much smaller inductors for a lower price, but than bigger capaciators....."

What makes Inductor EQs sound different?
Old 2nd August 2020
  #18
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Here's the schematic of the EQ I'm going to try transformers with. If I add input transformers, can I remove the capacitors on the inputs?
Attached Thumbnails
Adding transformers to consumer gear-screenshot_2020-08-01-20-47-43.jpg  
Old 2nd August 2020
  #19
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nosebleedaudio's Avatar
Quote:
Originally Posted by craigmorris74 View Post
Here's the schematic of the EQ I'm going to try transformers with. If I add input transformers, can I remove the capacitors on the inputs?
I would NOT..
Old 2nd August 2020
  #20
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Quote:
Originally Posted by nosebleedaudio View Post
I would NOT..
Yeah, that was a dumb question.
Old 2nd August 2020
  #21
Quote:
Originally Posted by Phil Cibley View Post
You could also try something like the Henry Engineering Matchbox which
is active but transformerless. Converts -10 unbalanced to +4 Balanced
and vice versa.
What I suggested in my #2 post...FWIW
Old 2nd August 2020
  #22
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Considering it runs on +43V it has GOOD headroom..
Old 3rd August 2020
  #23
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Quote:
Originally Posted by nosebleedaudio View Post
Considering it runs on +43V it has GOOD headroom..
This isn't a good assumption . . . the circuit can't swing anywhere near the full rail voltage. The limiting factor will be the negative swing on the output, and this will depend on numerous circuit tolerances, including the beta and cutoff voltage of TR107/108, tons of actual resistor values, and the raw line voltage (unregulated supply).

From just a casual glance, I'd say that 10 volts peak is about all you can count on . . . which corresponds to 7.1V RMS . . . and assuming a max boost of 12dB, that's 1.8V RMS on the input, or +7.2 dBu. This won't cut it in a +4 dBu studio environment, especially with no metering or input level control. Perhaps of even greater concern . . . the output is simply coupled off of the collector, so the loading impedance will directly affect the open-loop gain and response, and the leakage reactances of an output transformer could possibly send it into oscillation, depending on the setting of the sliders.

I'm not criticizing the design as a whole -- it's actually quite elegant for an inexpensive consumer EQ of the era . . . but this was a time where -10 dBV was considered the hot side of "line level" for consumer gear, and typical input impedance of the preamp/receiver would have been 100K or higher. Regardless of what the zero-signal, mid-band output impedance would measure to be . . . it's certainly not going to be comfortable driving a "balanced-bridging" pro input, let alone 600 ohms.

If this is going to live on a patchbay, then I'd have to change my mind and say that an active converter like the Matchbox is the best off-the-shelf solution, though its 10K input impedance is probably absolutely the lowest this EQ wants to see. The Matchbox's adjustable gain will allow it to be dialed in for the best trade-off in terms of noise floor and headroom, with good immunity to connection issues with other gear. If you really want an all-transformer, all-passive solution, then a 4:1 line input transformer is the way to go . . . and on the output side, perhaps use a DI transformer and run it back into a mic input to make up the gain?

Of course I'm coming at this under the assumption that you want an interface solution that's predictable, and delivers the sound of just the EQ itself without anything added, in all manner of situations. If on the other hand driving the thing into clipping is what you're after, then by all means, just do whatever . . . maybe start with just some adapter cables, and twiddle with stuff until you like what you hear.
Old 3rd August 2020
  #24
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Quote:
Originally Posted by kirkus View Post
This isn't a good assumption . . . the circuit can't swing anywhere near the full rail voltage. T"balanced-bridging" pro input, let alone 600 ohms.
You made an assumption...
I said nothing regarding maximum swing...
Just comparing it to MOST other -10dBV circuits of that time period that runs on anywhere from +12V to +24V..
Old 4th August 2020
  #25
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I was just referring to your statement that it had “GOOD headroom”, based purely on the B+ being 43 volts. Rather, the headroom is comparable to a design with rail-to-rail opamps running on a +/- 10 volt supply.

Also, any consideration of “headroom” has to be made with the unit’s controls in the worst-case setting to be considered valid, especially with no input controls, metering, or even a clip indicator. Putting together all the relevant factors ... for the intended application ... “GOOD headroom” cannot be assumed, and this should be factored into the interface plan.
Old 4th August 2020
  #26
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I finally received the EQ, and I ran some quick sweeps through it with and without transformers to see how it performed. I also ran some recently recorded drums through it, and I think it sounds fantastic.

On the graph I'm posting:
red-no transformers
green-10K:600 input transformer
orange-input transformer plus 600:15K output transformer

I also tried sending the output into a direct box into a mic pre, and the low and high end performance was better that with the output transformer, but no as good as without anything on the output.
Attached Thumbnails
Adding transformers to consumer gear-realistic-frequency-response.png  
Old 4th August 2020
  #27
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Quote:
Originally Posted by craigmorris74 View Post
I finally received the EQ, and I ran some quick sweeps through it with and without transformers to see how it performed. I also ran some recently recorded drums through it, and I think it sounds fantastic.

On the graph I'm posting:
red-no transformers
green-10K:600 input transformer
orange-input transformer plus 600:15K output transformer

I also tried sending the output into a direct box into a mic pre, and the low and high end performance was better that with the output transformer, but no as good as without anything on the output.
Thanks for doing this Craig. Can I ss which transformers you used? I picked one of these up this week because of this thread. I haven’t played with it much but so far it sounds great.

Btw does anyone else have trouble getting the slider knobs off? I think mine must be glued on.
Old 4th August 2020
  #28
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Quote:
Originally Posted by craigmorris74 View Post
I finally received the EQ, and I ran some quick sweeps through it with and without transformers to see how it performed. I also ran some recently recorded drums through it, and I think it sounds fantastic.

On the graph I'm posting:
red-no transformers
green-10K:600 input transformer
orange-input transformer plus 600:15K output transformer

I also tried sending the output into a direct box into a mic pre, and the low and high end performance was better that with the output transformer, but no as good as without anything on the output.
I would replace the output caps with at least 100uf BP.
Old 4th August 2020
  #29
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Quote:
Originally Posted by enginefire View Post
Thanks for doing this Craig. Can I ss which transformers you used? I picked one of these up this week because of this thread. I haven’t played with it much but so far it sounds great.

Btw does anyone else have trouble getting the slider knobs off? I think mine must be glued on.
I used an Edcor XSM 10k:600 on input, and the output was an old phone transformer that is flat from DC to as high as I can measure on the computer.

I haven't tried taking the slider knobs off yet.
Old 4th August 2020
  #30
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Quote:
Originally Posted by nosebleedaudio View Post
I would replace the output caps with at least 100uf BP.
Would that increasefrequency response in the low end?
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