Pultec filter section question

[Klaus Mogensen]

Damn, maybe i'll build the slowblow with the G9 PSU. That seems fairly straight forward.

Thanks for the answer.

 

[clintrubber]

[quote author="Klaus Mogensen"]I'm currently thinking about what to do with the output amp. Do any of you know whether it is possible to mod the Pultec PEQ1a amp to obtain 6 dB more gain without pressing it too hard (total 31 dB).

http://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif[/quote]

Are you using an output-transformer ? If so, and your loading is not too low (light enough), then another TX-ratio could give you that 6dB.

Other option is to add some pre-EQ-gain. While nobody has faced any real problems I understood, fact is that the input-Z of the EQ-section is low (can even be as low as 75 as Jakob pointed out here before). So the preceeding box needs to be able to deliver - or you might want to add a buffer, solid state being the most practical here.
You can - and now I'm finally coming to the point for this second option :wink: - add some gain in this buffer - as long as you're not saturating the EQ-inductors.

Bye,

Peter

 

[Klaus Mogensen]

Wiring the output transformer differently might endeed be the easiest solution, if this doesn't cause any additional problems. :thumb:

I, however, don't have much rack space, so I'm getting more and more keen on the idea of using the slowblow and making the unit so it can also be used as a normal microphone amplifier (using a Lundahl 1538xl input transformer). Then maybe also with the option of swicthing some of the eq boards in between the two gain stages for tracking, if this is possible considering the low input impedance of the eq boards.

 

[clintrubber]

[quote author="Klaus Mogensen"]Wiring the output transformer differently might endeed be the easiest solution, if this doesn't cause any additional problems. :thumb: [/quote]If you can get away with it there's also 6dB to be had from going out balanced (SRPP+TX-out) to the next box but there going in unbalanced. I mean, if that box is balanced at all, then connect the signal between + & GND i.s.o. between + & -. It might stink or it might work fine...

I, however, don't have much rack space, so I'm getting more and more keen on the idea of using the slowblow and making the unit so it can also be used as a normal microphone amplifier (using a Lundahl 1538xl input transformer).

I see, the modular approach. Should work fine.

Then maybe also with the option of swicthing some of the eq boards in between the two gain stages for tracking, if this is possible considering the low input impedance of the eq boards.

If your source can deliver it. The most easy ways out are (as been suggested somewhere here before): (1) by means of input-TX-ratio or (2) simple preceeding 553X-stage with 600 Ohm build-out-resistance.

Bye,

Peter

 

[Klaus Mogensen]

Maybe I should just stay with modding the original design using one of the two options mentioned by you, since I've almost got the boards stuffed anyway. :shock:
It will very fast become cumbersome if impedance conversion is needed between the amplification stages.
I often go from balanced to unbalanced without problems.

Thanks for the ideas

 

[CJ]

If you could live with a little less db cut and boost in the eq section, you could tweak the passive filter to lose less signal. Migh have problems getting a full 6 db, but 3 db would be easy.

 

[Klaus Mogensen]

Yes. I'll assemble the stuff and see what turns out the best. For mixdown I often use a limiter/compressor afterwards and can probably use the makeup gain from that one.
Maybe adding switches to turn the boards in and out of the circuit (and a volume pot) would be handy. Because often all three boards will not be necessary. :grin:

 

[cstella]

Hi I have been building up a pair of Gyraf Pultecs. One is finished and sounding great. I replaced the tube output stage with a Forssell 992.

I was wondering if it would be possible to change the High freq cut so that it was seletable between cut and boost. Any help would be greatly appreciated! Thanks! Charles.

 

[yosh]

I've been building a pair of the gyraf pultecs. I just finished them and they sound great, however, I want different frequency selections for the low boost and the low cut. It's my understanding that for the low frequencies the 2 poles of the switch are used to have one set of frequencies for boost and another set for cut. I think that I've figured out some aproximate cap values (in conjuction with the 10k resistor, yes?) for the low boost section, but I can't figure out what resistance I should use in my formula for the low cut. There's a 1k resistor there, but if I check my math for the existing cap and frequency values with the 1k it doesn't add up.

Could someone tell me what resistance value I should be using in my formula for the gyraf pultec srpp low cut?

Also, I want to replace my switches. I used non-shorting switches and I get pops as I change frequency selections. I assume that if I switch to shorting switches that would fix the problem. Is that correct?

I know lorlins are typically used as switches for this project, but are there any other suggestions? Do the lorlins last well, or are there others that are know to have a little more longevity?

Thanks!!

-Josh

 

[Purusha]

[quote author="JPrisus"]i'm thinking about adding the mid cut filter from an MEQ to the Gyraf Pultec EQP1 clone... is it possible to just 'drop in' the mid cut circuit somewhere in the Pultec schematic? If so, what makes the most sense? anyone try this before? It's pretty standard amongst the modern pultec-inspired EQs like the DW Fearn VT4 etc.

I'll probably be doing the entire filter section point-to-point and using the PCB for the power and makeup gain sections.


BTW, got my GSSL PCB stuffed and soldered in about 4 hours today... waiting on a voltage regular which was backordered, can't wait!!![/quote]

So has anyone actually tried this at the end? How does it sound?

 

[Luny Tune]

[quote author="yosh"]I want different frequency selections for the low boost and the low cut.

...but I can't figure out what resistance I should use in my formula for the low cut. There's a 1k resistor there, but if I check my math for the existing cap and frequency values with the 1k it doesn't add up.[/quote]
And neither should you care too much. You determine the frequencies with the caps. Leave the resistors in the cirquit as they are and just change the caps.

Just split the selector switch into two switches and you'll have cut and boost selection on two switches and the eq will work just fine like that.

You can check my topic about something similar here:
http://www.groupdiy.com/index.php?topic=17527

[quote author="yosh"]I know lorlins are typically used as switches for this project, but are there any other suggestions? Do the lorlins last well, or are there others that are know to have a little more longevity?[/quote]
Lorlin's are good quality switches with a nice..."click you can trust" to them... I know of Alpha switches too and the have a softer action and probably feels better with smaller knobs on them. How they are quality wise, I don't know. I have one sitting in one of my G1176 and it works just fine but it's just half a year old, so... :wink:

 

[Luny Tune]

[quote author="JPrisus"]i'm thinking about adding the mid cut filter from an MEQ to the Gyraf Pultec EQP1 clone... is it possible to just 'drop in' the mid cut circuit somewhere in the Pultec schematic?[/quote]
You will suffer double the gain loss that way which means you'll need an additional gain stage or a gain stage twice as powerful...

 

[Purusha]

Would 30dB be enough?

 

[Luny Tune]

I'm not absolutely sure but I believe that each of those passive filters takes the signal down by 21 dB, so it looks more like you'll need 42 dB of gain after the filter. BUT I'm certainly no expert on this. This is just my understanding from the answers I got when I asked the same questions some time ago. :wink:

 

[dagoose]

I'm also thinking of building the pultec that way, so far is haven't seen anyone actually building it, just some emulations.
Will it work, will it work good, how does it sound, what does the schematic look like? etc etc.

anyone?

 

[CJ]

look at the masiv p from m land for some interesting ideas.

 

[dagoose]

[quote author="CJ"]look at the masiv p from m land for some interesting ideas.[/quote]

where can i find that? a search didn't help me out..

 

[CJ]

...

 

[radiance]

Nah.. that's NYD's passive EQ design. Still have to make one of those when I find the time.

 

[CJ]

see here