[ruffrecords]

The OEP A187A15C is a 1:10 mic step up transformer. I am using it in the poor man's tube mixer because it is low cost. I am also using it in one version of the Mark 3 channel module. However, over the last couple of days of testing I have found some unexpected behaviour.

When driven from a low impedance generator  (less than 50 ohms) it exhibits the usual peaking response above 20Khz that you typically get with this 1:10 mic input transformers (I have seen it in Jensen, Sowter and Cinemags transformers).

When I insert my normal 20dB pad consisting of a pair of 680 ohm resistors in series with the hot and cold legs and a parallel 150 ohm resistor, the response flattens out right out to 40KHz (it does with Sowter, Jensen and Cinemag :10 ratio transformers). However, with this OEP transformer the response is actually down nearly a dB at 20KHz and nearly 5dB down at 40KHz. I spent a lot of time trying to find the cause for this but eventually tracked it down to the 20dB pad. The normal design with a 150 ohm parallel resistor works fine for all the other transformers. But with this OEP I found I needed to reduce it to 75 ohms to get the response flat past 20KHz. What concerns me is this implies the response will be less than ideal with regular 150 ohm mics.

Cheers

Ian

[5v333]

have you tried  loading the sec differently? RL and/or Zobel?

[5v333]

Sorry i think i had it wrong in myhead... again.

[gyraf]

I'm thinking excessive parasitic capacitance in the primary - possibly even between windings?

I think OEP keep prices down by applying a minimalist winding geometry scheme

And yeah, this leads to colors even at 150 Ohms mics..

/Jakob E.

[EmRR]

I’ve reported here on this phenomenon with pads before, to general disbelief.  More pronounced with 40dB line pads.   Not always present. 

[ruffrecords]

have you tried  loading the sec differently? RL and/or Zobel?

Yes I did. Originally I had the recommended 150K secondary resistor. I removed this and all that happened was the level went up by about half a dB because the tube stage has a very high input impedance. Like most modern mic input transformers there is no recommended Zobel network.

Cheers

Ian

[ruffrecords]

I'm thinking excessive parasitic capacitance in the primary - possibly even between windings?

At first I thought I might have forgotten to ground the inter-winding screens but I checked and triple checked and it was wired right.

I think OEP keep prices down by applying a minimalist winding geometry scheme

/Jakob E.

First I have heard of this. Do you have any evidence for it? I have emailed OEP about it so I will post their reply here.

Cheers

Ian

[gyraf]

No evidence for their minimalist approach - but judging from their transformers's general behavior I am pretty sure they don't do advanced interleaving and such. Possibly not even isolation between layers..

/Jakob E.

[5v333]

hmmm... the thing about loading could be worth some investigation still..

i have been staying inside a couple of days since ive been feeling alittle bit sick latly.
made some cleaning and organising of components on my work bench.

found a salvage input transfomrer from an old allenheath desk. DCR is 10R : 1k5. with 150R source and no load, the gain is about 17-18 and -3db is about 3.5Hz - 35kHz.

If i put 100k on the sec i loose a little bit of gain but get a bandwidth of about 45kHz. so a little bit of loading can traid some gain for bandwidth (to some degree i guess)

Have you tried a heavier load on the OEP? like 47k or something?


I have got the feeling that a transformer is happy with a certain load/impedance. the impedance is the total/paralell of pri source and sec load.

so if your source is not  quite low enough, you might be able to compensate to some degree with loading the sec.

does it sound like fact or fiction?

[ruffrecords]

Have you tried a heavier load on the OEP? like 47k or something?

I have not tried that. I was hoping for a reply from OEP by now but nothing in the in box today so I might try a lower secondary load. Thing is, for the primary to look like 1K5 you need a secondary lad of 150K. If I change that to 47K then the  primary looks like 470 ohms which may well mess with the frequency response of some mics. I will stick one in today and see what difference it makes.

Cheers

Ian

[5v333]

oh..
not good ofcourse.

[abbey road d enfer]

It definitely looks like a combination of too much leakage inductance/too much secondary capacitance.
I think you're equipped to measure both.

[ruffrecords]

It definitely looks like a combination of too much leakage inductance/too much secondary capacitance.
I think you're equipped to measure both.

I am and I shall. Good suggestion Abbey.

Cheers

Ian

[80hinhiding]

I have that transformer here and haven't had time to test it yet.
In the datasheet they loaded the secondary with 11k, source of 50ohms.  I thought ~11k would be used for 150ohm source but just a wild guess.  I'm going to apply this transformer to a solid state mic preamp eventually.

edit. Looking at some Cinemag mic input transformer documentation and some mic preamp circuits I see they have a RC snubber circuit in parallel with the RL of 150K, source of 150ohm.  The voltage gain measurement in the OEP datasheet for the A187A15C transformer is at 150ohm source, 150K RL.  The secondary total is listed at 1.59K and primary at 20.2R. 

Adam

[abbey road d enfer]

I have that transformer here and haven't had time to test it yet.
In the datasheet they loaded the secondary with 11k, source of 50ohms.

That is clearly not the way it is supposed to operate, which suggests they have chosen a setup that provides better figures.

I thought ~11k would be used for 150ohm source but just a wild guess.

11k at the secondary reflects as 150 ohms at the primary, which is not correct for customary n1:10 bridging. that would be correct for 1:1 matching, though.

edit. Looking at some Cinemag mic input transformer documentation and some mic preamp circuits I see they have a RC snubber circuit in parallel with the RL of 150K, source of 150ohm. 

That is quite usual; said Zobel circuit linearizes the HF impedance without loading at LF and MF.

The voltage gain measurement in the OEP datasheet for the A187A15C transformer is at 150ohm source, 150K RL. [/quote] That is more in line with typical usage; the reflected input impedance then is about 1.5k, which is adequate for 1:10 bridging.
But it shows that the specsheet is a tad devious, because if the gain measurement was done with the previous setup (50ohms source, 11k load, the gain would be only about 12dB.

Adam
[/quote]

[80hinhiding]

Cool cool, thanks.
Would 125K RL reflect as 1.59K on the primary in this case?

[abbey road d enfer]

Cool cool, thanks.
Would 125K RL reflect as 1.59K on the primary in this case?

A little less:
Zpri= Rpri+ (N1/N2)²[Rsec+RL]
That would be 20+127500/100=1295
This is neglecting several other parameters and tolerances that may shift the result by several percent.

[ruffrecords]

I have heard back from OEP. They are going to replicate my tests and let me know the results.

Cheers

Ian

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