The No.1 Website for Pro Audio
Adding transformers to consumer gear
Old 4th August 2020
  #31
Lives for gear
 

Quote:
Originally Posted by craigmorris74 View Post
Would that increasefrequency response in the low end?
Yes, increasing the size of the output capacitor will extend the low-frequency response.

But . . . without re-engineering the circuit, the only way to extend/preserve the high-frequency response is to operate the unit into a high(er)-impedance load . . . and this will also extend/preserve the low-frequency response at the same time. So I'd focus more on a solution to get the HF right . . . if you do this, the LF will come along for the ride.
Old 5th August 2020
  #32
Lives for gear
Quote:
Originally Posted by craigmorris74 View Post
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.
Maybe the poor performance of the output transformer in circuit vs on its own has to do with the zobel network? Or mismatched impedance?
Old 11th August 2020
  #33
Gear Addict
 
craigmorris74's Avatar
 

I don't think using an output transformer is going to be practical for me with this circuit. Could I use a THAT1646 to balance the output signal?

Last edited by craigmorris74; 11th August 2020 at 09:30 PM..
Old 12th August 2020
  #34
Lives for gear
 

Quote:
Originally Posted by craigmorris74 View Post
I don't think using an output transformer is going to be practical for me with this circuit. Could I use a THAT1646 to balance the output signal?
The main issue you'll have with using a balanced line driver IC like the 1646 or DRV134 is the fact that you only have one supply rail, and these ICs are designed for split-rail operation. Not that it's impossible to cap-couple the inputs and output and float them . . . but doing so is sufficiently "off-datasheet" to the point of maybe having some really unpredictable behavior.

If you want to add a simple circuit under the hood and run off the existing supply, I'd suggest taking a look at the attached diagram . . . please forgive the quick pencil sketch. Key to the operation are the fact that it must be DC coupled to the collectors of the EQ's existing output transistors, which is what sets the DC bias at around the midpoint of the supply rails. Just remove the existing output caps from the circuit, so as to render the existing 10K bleed resistors also out of circuit (the AC input impedance of the buffer is 10K to replace this). All of the diodes shown are to prevent latch-up as the supply comes up from 0v. It's also for this reason that the capacitor values are kept pretty small. Suggested opamp is the OP275 (one half for each stereo channel) for the medium-impedance noise characteristics, high supply rail tolerance, and availability in a through-hole DIP package. I did show a zener dropper to keep from pushing the voltage limits of the opamp, should the actual supply voltage is north of the stated "+43v" . . . but if it measures more on the low side of this in your unit with your local mains voltage, you'd probably be fine without it.

This gives what's frequently called an "impedance-balanced" or "pseudo-balanced" output . . . when operated into a halfway-well-designed balanced input, it gives the same noise rejection as the vast majority of active-balanced output designs out there. C5, C6, and XLR pin 1 should be connected to the chassis right at the connector, keeping the leads as short as possible . . . this is for RFI suppression. With the resistor values shown, the circuit gives about 12dB gain . . . the assumption is that you'd be using a 4:1 input transformer, so the result is about unity gain and appropriate headroom for a +4 dBu environment.
Attached Thumbnails
Adding transformers to consumer gear-single-supply-pseudo-balanced-out.jpg  
Old 12th August 2020
  #35
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by kirkus View Post
The main issue you'll have with using a balanced line driver IC like the 1646 or DRV134 is the fact that you only have one supply rail, and these ICs are designed for split-rail operation.
Thanks for your reply!

I should have been more clear. I intended to use the 1646 in a separate box with it's own +/-16V power supply.
Old 12th August 2020
  #36
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by nosebleedaudio View Post
I would replace the output caps with at least 100uf BP.
Thanks for this recommendation. I did this, and now it's only down 0.2dB at 10Hz!
Old 16th August 2020
  #37
New Cap

What did the LF response look like before the recap?

I find the bass to feel big and puffy--not sure if that has anything to do with the cap or the Q of the 60hz band.
Old 16th August 2020
  #38
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by ShardsLV View Post
What did the LF response look like before the recap?

I find the bass to feel big and puffy--not sure if that has anything to do with the cap or the Q of the 60hz band.
See the graphs in post 26.
Old 16th August 2020
  #39
Gear Guru
 
Karloff70's Avatar
 

How funny, I just got one of these recently and was also looking to put trannies in it, but Mick at DAV reckoned it wouldn't happily drive them so I didn't. Also, it just sounds great as is and there is enough juice coming out.

Interesting about the output caps opening up the bottom that much, but I definitely won't do that to this one as I love the way it tightens up things at the moment. And yes, you can change the frequency by swapping a cap, but the Q changes with it so not too far. Mick changed my 3.5kHz to 2.8 for a 'wannabe Helios boost' lol. Love it.

Also just got a 1988 (for 25 quid hahaha), thinking it should sound the same, as seems pretty identical bar the better noise spec. But it isn't quite as magical/glowy. Might get another 87. Put the big out caps in that one.
Old 17th August 2020
  #40
Lives for gear
Quote:
Originally Posted by Karloff70 View Post
How funny, I just got one of these recently and was also looking to put trannies in it, but Mick at DAV reckoned it wouldn't happily drive them so I didn't. Also, it just sounds great as is and there is enough juice coming out.

Interesting about the output caps opening up the bottom that much, but I definitely won't do that to this one as I love the way it tightens up things at the moment. And yes, you can change the frequency by swapping a cap, but the Q changes with it so not too far. Mick changed my 3.5kHz to 2.8 for a 'wannabe Helios boost' lol. Love it.

Also just got a 1988 (for 25 quid hahaha), thinking it should sound the same, as seems pretty identical bar the better noise spec. But it isn't quite as magical/glowy. Might get another 87. Put the big out caps in that one.
Do you know what value cap he used? I’m trying to get an idea of how to mod inductor eqs, which direction the frequency shift happens etc.
Old 17th August 2020
  #41
My bad--didn't see it! Really cool to see ppl digging into this eq. Stock it's rad. Just used it today on a session to make some clean ld guitar mid-punch an 1176. You can crank it!
Old 17th August 2020
  #42
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by ShardsLV View Post
My bad--didn't see it! Really cool to see ppl digging into this eq. Stock it's rad. Just used it today on a session to make some clean ld guitar mid-punch an 1176. You can crank it!
You can crank the heck out of it and it still sounds great!
Old 17th August 2020
  #43
Gear Guru
 
Karloff70's Avatar
 

Quote:
Originally Posted by enginefire View Post
Do you know what value cap he used? I’m trying to get an idea of how to mod inductor eqs, which direction the frequency shift happens etc.
Nope, sorry, no idea. He just said the Q changed/got a little wider by moving to 2.8 and the more you change frequency with cap size alone (not changing inductor), the more that would happen. Or, that's how I understood it.
Old 17th August 2020
  #44
Lives for gear
Quote:
Originally Posted by Karloff70 View Post
Nope, sorry, no idea. He just said the Q changed/got a little wider by moving to 2.8 and the more you change frequency with cap size alone (not changing inductor), the more that would happen. Or, that's how I understood it.
Your explanation is good, it makes a lot of sense. I’m sure there is a formula to figure it out, I’m just not sure what it is. It would be nice to add some three way switches to be able to change each band a little up or down, even if that also adjusts the Q a bit. Options
Old 17th August 2020
  #45
Gear Guru
 
Karloff70's Avatar
 

Quote:
Originally Posted by enginefire View Post
Your explanation is good, it makes a lot of sense. I’m sure there is a formula to figure it out, I’m just not sure what it is. It would be nice to add some three way switches to be able to change each band a little up or down, even if that also adjusts the Q a bit. Options
Not a bad idea. For a handful of caps and mini switches, certainly not much cost, and not too much hassle either me thinks. Might look into that. :-)
Old 17th August 2020
  #46
Lives for gear
Quote:
Originally Posted by Karloff70 View Post
Not a bad idea. For a handful of caps and mini switches, certainly not much cost, and not too much hassle either me thinks. Might look into that. :-)
If it’s like a gyrator eq circuit (the op amp simulator of inductor eqs), then increasing the cap should decrease the fréquence, and decreasing the cap value will increase (raise) the center frequency.

From what I could tell from looking into the side of the faders, it looked like it was using tant caps. I’m not sure though, my fader knobs seemed glued on to the faders, they wouldn’t pull off at all.

Let me know if you have the same experience!
Old 20th August 2020
  #47
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by enginefire View Post
If it’s like a gyrator eq circuit (the op amp simulator of inductor eqs), then increasing the cap should decrease the fréquence, and decreasing the cap value will increase (raise) the center frequency.

From what I could tell from looking into the side of the faders, it looked like it was using tant caps. I’m not sure though, my fader knobs seemed glued on to the faders, they wouldn’t pull off at all.

Let me know if you have the same experience!
I started to take apart my 31-1987, and the knobs slipped right off. I'm replacing the faders with rotary pots to fit it and a 31-1988 in a 2u rack.

Also, Ian, who posts here sometimes suggested installing 10k:10K transformers and resistors for impedance balancing to overcome the -6db loss in signal, which I was getting with mine.
Old 20th August 2020
  #48
Lives for gear
This sounds like it will be a great project. My knobs bits have been crazy glued in place. In the 2u you probably will have enough room to add the electronically balanced output!
Old 20th August 2020
  #49
Gear Guru
 
Karloff70's Avatar
 

Quote:
Originally Posted by enginefire View Post
This sounds like it will be a great project. My knobs bits have been crazy glued in place. In the 2u you probably will have enough room to add the electronically balanced output!
Hmmm, only all the ready made balancing boards like JLM and others are basically around chips. I think I would rather just keep using these like they are instead of putting chip amps before and after it.....

If anything I would put transformers on the output if it can drive them, as to get a two legged no level loss output. And just wire the inputs on one leg. That way the transformer would eat some interference as well.

But, to be honest, patching these into a conversion loop I don't really get enough loss to not be able to use them easily as they are, so I'll need to see some seriously cool pics and hear awesome stories of much better sound in some way to stop running these ghetto and do work on them
Old 21st August 2020
  #50
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by Karloff70 View Post
Hmmm, only all the ready made balancing boards like JLM and others are basically around chips. I think I would rather just keep using these like they are instead of putting chip amps before and after it.....

If anything I would put transformers on the output if it can drive them, as to get a two legged no level loss output. And just wire the inputs on one leg. That way the transformer would eat some interference as well.

But, to be honest, patching these into a conversion loop I don't really get enough loss to not be able to use them easily as they are, so I'll need to see some seriously cool pics and hear awesome stories of much better sound in some way to stop running these ghetto and do work on them
I've decided to try 10k:10k transformers on the inputs. They won't drive output transformers of any reasonable value.
Old 21st August 2020
  #51
Lives for gear
 
nosebleedaudio's Avatar
Quote:
Originally Posted by craigmorris74 View Post
I've decided to try 10k:10k transformers on the inputs. They won't drive output transformers of any reasonable value.
Not all Output transformers 1:1 600 Ohm present the same 'Load" on the amp driving them..
I was testing several a few weeks ago and noticed this more closely..
You also need to know for sure Where the drop in level is & why if possible, one thing for sure is the power supply is NOT regulated..
Old 21st August 2020
  #52
Gear Guru
 
Karloff70's Avatar
 

Quote:
Originally Posted by nosebleedaudio View Post
Not all Output transformers 1:1 600 Ohm present the same 'Load" on the amp driving them..
I was testing several a few weeks ago and noticed this more closely..
You also need to know for sure Where the drop in level is & why if possible, one thing for sure is the power supply is NOT regulated..
Ok, so making a better psu could make these sound even better. Personally not sure input trannies would do much for the trouble.
Old 21st August 2020
  #53
Lives for gear
It would be interesting to see how a regulated supply changes the sound. Some of the character might be coming from the psu sagging a bit when it’s pushed.
Old 21st August 2020
  #54
Gear Guru
 
Karloff70's Avatar
 

Maybe. I hear quite a vibe difference between the 1987 and the 1988 here. The 87 being rather vibier, and the 88 a little more dry feeling. Specs show noise difference but not much else.....
Old 21st August 2020
  #55
Lives for gear
 

I think it might be helpful to consider some of the factors that the original designers were working with, in considering mods to this unit . . .

First, it was built in a rather unique period of consumer electronics design where opamps were still relatively expensive, discrete transistors were very cheap, and engineers still had lots of familiarity and experience with classical tube circuits. Japan in the mid-20th century also had very low labor costs compared to their high skill in manufacturing certain precision components, including transformers, inductors, phono cartridges . . . that is, small-signal audio stuff with windings. Thus, they were able to make a low-budget equalizer with L-C components, keeping costs low by having only a handful of bands, and very few active stages. The latter is responsible for the issues with driving output transformers and lower-impedance pro gear . . . but this was not a problem for consumer inputs of the era.

Most "more sophisticated" consumer units of this era had ten bands, and the biggest cost of doing this is the bigger enclosure and more sliders, and double the number of inductors. This tipped the cost equation to the point where it made sense to use transistor gyrator circuits to emulate the inductors. Since a gyrator can only emulate a grounded inductor (not a floating one), the summing circuits had to become more complex, as well as more numerous to prevent interaction between the (narrower) adjacent bands. As opamps became better and cheaper, it only improved the economics of the equation when using gyrators, and these market forces are also making inductors much less in general demand, so what was a pricey part gets even pricier.

So an inexpensive consumer equalizer based around an LC design is a pretty rare bird, even as pro units were commonly produced well into the 1980s.

Quote:
Originally Posted by enginefire View Post
If it’s like a gyrator eq circuit (the op amp simulator of inductor eqs), then increasing the cap should decrease the fréquence, and decreasing the cap value will increase (raise) the center frequency.
A good description of these L-C equalizer circuits can be found in the manual for the White 4000 series on pages 6-8. Note that this topology requires an inverting amplifier, which is why the Realistic unit inverts polarity. As far as changing the frequency of the bands goes . . . there's really no way to do it without changing the values of the inductors. Merely changing the caps will alter the Q as well . . . i.e. raising their values will both shift the frequency of the band up, and lower the Q. Lowering their value will increase the Q and shift the band downwards . . . but the associated circuit has limited loop gain and bandwidth, which means that there's a limit to how high the Q can be, and it could also become prone to oscillation. Since the Q also varies with slider position (higher Q at more gain), any stability problems will also be dependent on the aggregate settings across all the bands. In theory, one could change the frequency by keeping the L constant and varying both the C and the resistor in series with the L-C circuit, but will affect the maximum boost/cut level. That could be workable, if it was done across ALL the bands, within the confines of the impedances of both the sliders and the the amplifier circuit . . . but this is going pretty far down the road of re-engineering the whole thing.

Quote:
Originally Posted by nosebleedaudio View Post
Not all Output transformers 1:1 600 Ohm present the same 'Load" on the amp driving them..
I was testing several a few weeks ago and noticed this more closely..
You also need to know for sure Where the drop in level is & why if possible, one thing for sure is the power supply is NOT regulated..
This is a class A circuit, so there's a constant load on the power supply regardless of the signal level. But I absolutely agree that in order to come up with a suitable interface plan, one first has to investigate and understand the specific conditions that are causing any signal loss, whether frequency-independent or frequency-dependent.

From the schematic, it's obvious that it has a high output impedance . . . and it's likely that if the graphs in post 26 were made using a modern sound interface, that even the interface's input impedance (10K?) was low enough to get some significant loss. My suggestion would be to measure the 1KHz output with only a DMM connected, and add various resistors in parallel to see how much voltage drop you get. You can then figure out the output impedance . . . i.e. if adding a 10K resistor in parallel drops the output by 6dB, that's half the voltage, meaning the unit's output impedance is also 10K.

If you don't have a DMM, you could also use a high-impedance active DI (i.e. BSS AR-133, Countryman Type 85) for the same test, and even run a frequency sweep . . . though keep in mind that these DIs still aren't perfectly flat, so sweep it by itself so you can subtract the DI's LF and HF rolloff characteristics from the unit under test.

FWIW the buffer circuit I posted earlier will perfectly drive just about any 1:1 line output transformer installed before the XLR output. Tiny optimizations for such would be to change R6/R7 to 100K, R4/R5 to 39 ohms, and C3/C4 to 220uF. If you don't need the 12dB gain (i.e. a 10K:10K transformer is on the input) R3 can be 10K, C2 220pF.
Old 4 weeks ago
  #56
Gear Addict
 
craigmorris74's Avatar
 

I tried my 10k:10k input transformers (Edcor). Oddly enough, when wired as intended, I still get a loss in level, but when wired backard, it cures the level loss problem.
Old 4 weeks ago
  #57
Gear Guru
 
Karloff70's Avatar
 

Quote:
Originally Posted by craigmorris74 View Post
I tried my 10k:10k input transformers (Edcor). Oddly enough, when wired as intended, I still get a loss in level, but when wired backard, it cures the level loss problem.
Backward? How do you wire a 1:1 tranny backward?
Old 4 weeks ago
  #58
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by Karloff70 View Post
Backward? How do you wire a 1:1 tranny backward?
The spec sheet from Edcor says pins 1 and 3 are the input and 5 and 7 are the output. It didn't work that way.
Old 4 weeks ago
  #59
Lives for gear
 
nosebleedaudio's Avatar
Quote:
Originally Posted by craigmorris74 View Post
The spec sheet from Edcor says pins 1 and 3 are the input and 5 and 7 are the output. It didn't work that way.
Something sounds wrong; If it is a basic 600 Ohm to 600 Ohm (1:1) line output transformer it "Should" work either way..
But it IS recommended to use a certain winding for Input..
IM not sure if this is the 10K to 10K line in Or an Output transformer..
Old 4 weeks ago
  #60
Gear Addict
 
craigmorris74's Avatar
 

Quote:
Originally Posted by nosebleedaudio View Post
Something sounds wrong; If it is a basic 600 Ohm to 600 Ohm (1:1) line output transformer it "Should" work either way..
But it IS recommended to use a certain winding for Input..
IM not sure if this is the 10K to 10K line in Or an Output transformer..
You're right, when I tested it the last time, I had it connected wrong. It works fine now.I

So if you're losing 6dB running signal through this EQ, a 10k:10k transformer will fix the issue.
📝 Reply
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
🖨️ Show Printable Version
✉️ Email this Page
🔍 Search thread
🎙️ View mentioned gear
Forum Jump
Forum Jump