[BUILD] fripholm's TG1 Zener Limiter boards - support thread

[dmp]

a good resource about transformer coupling?

There are probably many threads around.  I wrote quite a bit in this thread while building a high gain tube preamp (V76):
https://groupdiy.com/index.php?topic=68498.msg871733#msg871733

Main ideas are:
A magnetic field is generated in a power transformer from alternating current and a changing magnetic field can induce a signal in an audio transformer.
An ideal transformer would keep the magnetic flux fully contained in the core, but in reality the magnetic field leaks and propagates out around it.
Square EI type transformers will have higher outward magnetic fields than toroid types due to the geometry. If two EI transformers are near each other, minimize coupling by separating them as far as possible and orienting them at right angles (see the orientation of the power & output transformers on a '60s fender guitar amp). The separation distance is very effective since the strength of the magnetic field decreases with the inverse square law.
Using a toroid power transformer in a preamp / compressor etc is almost always a good idea, due to the low cost and low magnetic field leakage of a toroid.
Shielding can block a magnetic field by blocking (absorbing) it. The shielding is best located around the audio transformers to block a magnetic field from entering, but can also be around a power transformer to block the field from exiting.  The most effective shielding is mu-metal because of the high permeability. A shield must be continuous to allow the magnetic flux to fully loop through the shield (magnetic fields will always form complete loops). The magnetic flux is the field * permeability (B=mu*H). Note that emf shielding is not the same as magnetic shielding. Copper tape like used in guitar cavities is not effective to block magnetic fields. 

 

[JMan]

A magnetic field is generated in a power transformer from alternating current and a changing magnetic field can induce a signal in an audio transformer. 

dmp, this whole thing was a beautifully clear and fascinating explanation.  Thank you for taking the time!  I definitely want to continue reading on the subject, but your summary really seems to cover a lot of what I need to know for the practical purposes at hand.

One question — an AC power transformer can induce a signal in an audio transformer without proper distance and/or shielding (I actually am planning on both; power toroidal and audio transformers will be situated at more or less opposite ends of the enclosure, and I have a mu-metal can here that will fit my toroidal, so I’m going to put it in there).  Can the audio transformers do the same to one another?  Do I need to be particularly mindful of the input/output transformers’ arrangement relative to each other? 

This stuff is so interesting.  Once again, thanks for your very digestible answers, and your patience with my lack of knowledge!

 

[dmp]

One question — an AC power transformer can induce a signal in an audio transformer without proper distance and/or shielding (I actually am planning on both; power toroidal and audio transformers will be situated at more or less opposite ends of the enclosure, and I have a mu-metal can here that will fit my toroidal, so I’m going to put it in there).  Can the audio transformers do the same to one another?  Do I need to be particularly mindful of the input/output transformers’ arrangement relative to each other? 

Well, they can - but the signal levels are so much lower that there is much less magnetic field. At the signal transformers you may at most have 1.5 V to generate a field while the power transformer has 120vrms. So the signal transformers are much more susceptible to picking up a magnetic field than giving one off.
Secondly, any coupling via the magnetic field is 60 Hz hum when it comes from the power transformer, while it would be audio signal from the output to input transformer (so it wouldn't manifest as noise or hum). It may not even be a noticeable issue to have some audio coupling between the input and output transformers unless it was strong enough to cause oscillations.
For the cleanest build, place the input and output transformers as far apart as practical (but it is more important to have both the input and output as far as possible from the power transformer) and place the input and output transformers at right angles to each other and the power transformer. 



No problem - you're welcome

 

[HerbertR]

Hi, I am also starting the Build the next days.
Can anyone agree if the Carnhills VTB9046(M) and VTB2281 are working properly? Or are there any other recommandations? BR

 

[JMan]

Hi, I am also starting the Build the next days.
Can anyone agree if the Carnhills VTB9046(M) and VTB2281 are working properly? Or are there any other recommandations? BR

Hey, I haven’t quite gotten to the point of firing it up, but those are the transformers that have been recommended to me and to others in this thread.  I know that at least a few people have had successful builds with them.  I personally went back and forth quite a bit, but in the end, I ordered the Carnhills too.  As I said, haven’t quite gotten through everything yet, but as far as I understand these work just fine.

 

[BerndVP]

Hi,

Is this one ok for the build ?
https://benl.rs-online.com/web/p/toroidal-transformers/1233098/

 

[JMan]

Hi,

Is this one ok for the build ?
https://benl.rs-online.com/web/p/toroidal-transformers/1233098/

That’s the one!

 

[tommia]

I have a question conserning troubleshooting:
Does the unit work if the attack knob is not connected?
Is there a voltage chart for T11,Q8, D5 when not compressing?

@ bernd vp I have not build it but you need 28v dc so you need theoretically 25 v ac,  the unit on my workbench consumes les than 10 watt so 60 va is also overkill but it is the only option at rs

 

[JMan]

I have not build it but you need 28v dc so you need theoretically 25 v ac,  the unit on my workbench consumes les than 10 watt so 60 va is also overkill but it is the only option at rs

The BOM lists a toroidal transformer with 2x30v secondaries, which is why the one that Bernd listed has been recommended previously in this thread.

With regards to your question about the attack knob, are you asking whether you can set a single fixed attack time, or are you asking if the unit will work if the attack connections are broken entirely?

 

[fripholm]

Tommia is right - 2x25V would suffice but oftentimes 2x30V (and higher wattage than necessary) is the only option.

This has been mentioned before but if ordering from Reichelt is an option for you, this one is slightly less bulky and works great in my units.

Does the unit work if the attack knob is not connected?
Is there a voltage chart for T11,Q8, D5 when not compressing?

The circuit won't work without at least a fixed resistor in place of the attack control.

The following values were read from Spice without input signal and HOLD mod and should be pretty close to real world voltages:
Base Q8, Collector T11: ~ 13V
Base T11: ~ +200mV
Emitter T11: ~ -300mV (depends on trimmer FT2B or R56 if you have an older board revision)
Cathode D5: ~ -5.7V (depends on trimmer FT2B or R56 if you have an older board revision)

 

[BerndVP]

Hi,
Reichelt is an option for me 

waiting for the PCBs to arrive.

Finally finished the Front panel

 

[tommia]

Thanx for the quick answers. I'll get back to my workbench tomorrow.
I ordered some original zeners,  When they arrive, I can do some measurements if somebody is intersted.

 

[BerndVP]

Hi fripholm,

I just ordered all the components for the project.
Just 1 thing I just noticed and I'm not really know if this one is ok:
https://www.mouser.be/ProductDetail/652-PDB185GTR01254A2

It's for the Release / auto release and it's rated at 50 mW

 

[fripholm]

Yes Bernd, that should do.

 

[BerndVP]

Just saw the 56R for R75 has to be 1W, will an 0.6W work too ?

 

[fripholm]

Yes.

 

[tommia]

Thanks for the answer, just what I needed. One more successful build. Please use a lot of steps for all the switches, you won't regret the cost. I love the hold function. I used a potentiometer there. You expect that a compressor makes loud things quieter, but with this function you can make quiet things louder! This is a great compressor. Well matched diodes have a big influence on the noise ratio.

 

[JMan]

One more successful build.

Congrats!

I'm jealous, actually -- I've had to put the build on pause while I wait for some backordered bits and pieces...argh!

 

[tommia]

Who uses those mostly in stereo and who in 2 mono compressors?
I'm still deciding my enclosure: 2 linkable mono's or 1 stereo compressor

 

[andow]

Who uses those mostly in stereo and who in 2 mono compressors?
I'm still deciding my enclosure: 2 linkable mono's or 1 stereo compressor

Definitely stereo. This thing excels on overheads and room mics!