Pultec in/out transformer Impedance???

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nacho459

Well-known member
Joined
Jul 20, 2004
Messages
339
Location
Pasadena CA
What impedance transformers should I use for the input and output of my Pultec? 600?, 10k?? I really don?t understand line level impedance.

I like the sound of Jensen transformers.

I used these transformers I my LA-2a.
http://www.jensentransformers.com/datashts/11p1.pdf
http://www.jensentransformers.com/datashts/10k611m.pdf

What do you think? I am really clueless.
 
[quote author="cjenrick"]600:600 in,
any plate to line out.[/quote]

Can I use Beyerdynamic TR/BV 3.10.0.01.081, 1:1 - 1,2K line transformer.
For out I have LL5402
Duka
 
There is no need to use a 600:600 transformer at the input of this (or almost any) equipment. The gain stage driven by the transformer will nearly always have an input impedance greater than 10 k-ohms. This allows the use of a 1:1 turns ratio transformer (where primary input impedance is essentially the same as the load impedance on the secondary ... i.e., the gain stage input impedance) and results in a balanced "bridging" input impedance at the primary (input terminals). IF a true 600-ohm input impedance is desired for some reason (and these are rare), add a 620-ohm resistor across the primary (input terminal) side. The Jensen JT-11P-1 is an excellent 1:1 input transformer for this purpose and its internal Faraday shielding gives it a noise rejection (CMRR) figure of around 100 dB.

The "600 ohm" mentality just persists (from a long history in telephone equipment and subsequent tube electronics) in spite of the fact that the need for it is all but completely obsolete.

Bill Whitlock, president, Jensen Transformers, Inc.
 
But as an execption, on the pultec filter circuit, you will need a quite low-Z input transformer, because this passive eq circuit loads the source with down to 75 Ohms worst-case (that is, at max. HF boost, highest "Q")

I find the LL5402 (line output transformer) working really well here - but just about any transformer rated 600:600 will do (providing they're decent quality)

Jakob E.
 
[quote author="gyraf"]But as an execption, on the pultec filter circuit, you will need a quite low-Z input transformer, because this passive eq circuit loads the source with down to 75 Ohms worst-case (that is, at max. HF boost, highest "Q")

I find the LL5402 (line output transformer) working really well here - but just about any transformer rated 600:600 will do (providing they're decent quality)

Jakob E.[/quote]

While the combination of high levels at that freq & those settings will be rare, I'm still a bit worried for the gear that's before the EQ.

With a 1:1 or no TX, that worst case 75 Ohms load is one to one presented to the box driving the EQ. I guess that could be a bit demanding for at least my sources.

I'm thinking of toying around with that ~class A hybrid that'is used in 'a certain EQ-box of impressive dimensions' (OPA2604, MJ2955, MJ3055). That one is reported to be able to drive the worst case load of that certain EQ (100). Other buffers could of course be used, but for grins I want to try that one.

Anyone toyed around with that circuit perhaps ?
I've done some simulations so far & will breadboard, will post in sep. thread.

Regards,

Peter
 
Easy solution:

Use a 4:1 input and add an additional 12dB of gain at the output.

The lower input level with help to avoid saturating the inductors, but that could be part of what people like about the sound of those boxes, so "better" is a matter of opinion.

Theoretically better solution:

Use a low-Z buffer before the filter section. The source Z of the buffer will have to be very low, if you want max "boost" from the filters. If you were going the tube route, you'd pretty much have to use a WCF.

For what it's worth, Doug Fearn told me a few years ago that he drives the filter section in his Pultec with a cathode follower.
 
Thanks for the response Dave. :thumb:

[quote author="NewYorkDave"]Easy solution:

Use a 4:1 input and add an additional 12dB of gain at the output.[/quote]
This is also what the &a*l%y-doc told they did at first: their initial circuit was a 3:1 but since they had to cope with much higher losses for their EQ apparantly that lost 10dB was too much.

So they then did something like this:
driver_02_45.jpg


The lower input level with help to avoid saturating the inductors, but that could be part of what people like about the sound of those boxes, so "better" is a matter of opinion.
Have been wondering about that one as well - I saw another suited circuit that could have some gain in it as well
(rightmost part of http://sound.westhost.com/p24_fig1.gif ) but then started wondering about when things might go wrong 'coilwise', just like you said.

Theoretically better solution:

Use a low-Z buffer before the filter section. The source Z of the buffer will have to be very low, if you want max "boost" from the filters.
Want to do it like that. The shown circuits or Jakobs headphone-amp all look suited.
Also have been thinking of using 1731 or 2*5*2*0 since I have those PCBs available anyway, but it'll be better to keep this input-buffer simple & clean I figure.

If you were going the tube route, you'd pretty much have to use a WCF.

For what it's worth, Doug Fearn told me a few years ago that he drives the filter section in his Pultec with a cathode follower.

I was thinking of making them as passive sections first, so at first without the SRPP. Once I add that I could also add your suggested pre-EQ stage.

Thanks again,

Peter
 
OK, previous circuits could then be combined into something like this:
healthy bias, servo and (cheapo) balanced input.

Any comments ? Please shoot.

driver_03_45.jpg


A few things:

#1

I can't see the ESP-mentioned 80mA BJT-current.

Assuming thermal matching of the diodes & BJTs, one would expect
the BJT-current to be 68/4.7 times the resistive divider-current,
so around 20mA. Simulation confirms this (~26mA). Likely the 10k-resistor
need to be lower (or the 68R resistors higher) in value.


#2

I've increased the servo-cap, but have some mixed feelings about this
servo-topology itself. Too simple or just simple enough ?



Any thoughts ?


Thanks,

Peter
 
Having looked at some more alike circuits as that one here directly above I saw there are some possible improvements.

Apparently to increase the impedance above and under the two diodes I've seen current sources being used i.s.o. those 10k resistors (R1, R2).

Another way, but assuming to be aiming for the same: Jakobs headphone-driver uses bootstrapping, making those resistors higher-Z for AC.

I'll do some more thinking why a higher-Z is desirable there.
 
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