Pultec circuit between Neve BA283 in/out sections?

[jordan s]

I recently picked up some Neve BA283 cards locally from a tech parting out a board. I bought the transformers and all the parts I needed to rack up a pair of 1272 pres from these while simultaneously planning my build for a Poor Man's Pultec. After noticing that the BA283 output section has been used for make up gain on Pultec style circuits, I got the clever or stupid idea to make attempt to build my 1272 pres with the Pultec inserted between the input and output sections of the BA283. Now, from reading up on SS Pultecs, I know they can be extremely effected by input impedance. I really don't know what to expect when feeding the Pultec from the input section of the BA283 driving a mic signal. Before I dive head first and make a lot of stupid mistakes, any suggestions or comments would be very helpful and appreciated. Thanks.

 

[ruffrecords]

The input amp part of the BA283 should have no problem driving the Poor Man’s Pultec. I am not so sure about the output stage acting as gain make up for it though. The poor man's Pultec expects the gain make up amp to have a relatively high input impedance (200K or higher). A lower input impedance reduces the amount of lo boost and tends to exaggerate the lo cut. A simple one FET buffer may be all you need to fix this.

Cheers

Ian

 

[alexc]

Just on that subject, Ian do you have an idea of what the input impedance of the BA282 2nd stage might be?  I'm wondering what kind of source it would like to see.

I've seen it driven by the Neve 1st stage from the wiper of a 5K pot, where the pot has a 5K series resistance to ground.

BTW I found a preamp with pultec works well with a 1290 style setup

mic traffo/DI with pad -> neve 1st stage -> pultec -> neve 2nd stage -> gain pot -> neve line amp

This ends up with a gain structure similar to a 1272.

Without the extra stage you will miss the gain, even though you can 'pump up' the line amp to provide a substantial amount of gain itself.

 

[ruffrecords]

Just on that subject, Ian do you have an idea of what the input impedance of the BA282 2nd stage might be?  I'm wondering what kind of source it would like to see.

I've seen it driven by the Neve 1st stage from the wiper of a 5K pot, where the pot has a 5K series resistance to ground.
Without the extra stage you will miss the gain, even though you can 'pump up' the line amp to provide a substantial amount of gain itself.

From a quick look at the schematic I reckon it is no more than 56K so using the old 10:1 rule a 5K source would be fine.

Which means it is rather too low for the Poor Man's Pultec. I think the FET buffer is going to be required.

Cheers

Ian

 

[alexc]

Thanks Ian

That's been bugging me for a while.

I'm actually using it as a gain make up in a balanced summing bus. I have a few different transformers to sum into and so I wanted an idea of what might be best.

Cheers

 

[jordan s]

Thanks for the input guys. Ian, is there a specific fet buffer schematic you could recommend?

 

[alexc]

You might also find this thread interesting - similar subject

http://www.groupdiy.com/index.php?topic=43413.0

 

[ruffrecords]

Thanks for the input guys. Ian, is there a specific fet buffer schematic you could recommend?

We discussed this in the original drawing board thread. The relevant part starts here:

http://www.groupdiy.com/index.php?topic=43859.msg563662#msg563662

Cheers

Ian