HJFP- question about capacitors and transformers

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electropoet
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HJFP- question about capacitors and transformers

Post by electropoet » Fri Oct 05, 2007 11:25 am

Hi!

I'm building a few channels of HJFP on MNATS pcb which exactly follows Hamptone's schematic. The only difference is that it doesn't have phase (flip).

So, in my build i'm using capacitors instate of transformer for the output, same as suggested by Hamptone in his Tape Op article.
Suggested capacitor is polypropylene, but another source (Diy Factory) used tantals there.
Did anyone tried tantals vs polys? Or capacitor vs transformer coupling?
I know i could try capacitors myself, but i don't want to destroy traces on the pcb by heating them several times.

Thanks!

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Post by honkyjonk » Fri Oct 05, 2007 11:34 am

Hmmm. I'm not sure how a capacitor would go in place of the transformer in the, but maybe you need to put some extra caps in the cicuit somewhere if you're going transformerless for the output?

Anyway, probably most folks would reccomend using a transformer if you can.

But concerning the difference between tants and films, I think most DIY folks generally prefer a poly film cap to a tant in most circuits, if you can fit it in there.
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electropoet
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Post by electropoet » Fri Oct 05, 2007 11:40 am

Yes, you have to put two capacitors instate of transformer to the output. I've seen this practice many times.
Btw, the difference between cap and tx output is that you can't drive very low impedance with the cap. But all modern gear use 10k inputs, so i don't see the problem here.

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Post by rockstudio » Sat Oct 06, 2007 9:35 am

I think the schematic suggests that you may use your preamp with or without output transformers. You must have the coupling capacitors at the end of the gain modules to block dc between stages and the output. There should be around +12vdc before each coupling cap. Hamptone's article says that you may add an output transformer, however it is not necessary. I have two channels with output transformers, and one channel without them. They both work fine and sound fairly similar. I used Solen metalized Poly caps on all three of my channels.

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Post by philbo » Sun Oct 07, 2007 1:16 am

Poly caps are by far better. Tantalums may work, but must be tested to verify their effect on the sound (THD & intermod over dynamic range would be good things to check), since the dielectric absorption properties vary with the brand, size, body type and a few other things. This can, in some situations compormise the sound.

If you arent't in a situation where you could do this, it'd be much simpler to just get poly caps.
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Post by brianroth » Sun Oct 07, 2007 10:51 pm

While MOST "modern" gear does indeed have a relatively high bridging input impedance, you cannot make that assumption. The UA reissued gear comes to mind. And, tons of vintage gear did indeed have a 600 Ohm input impedance.

Hence, the output coupling cap needs to be at least 100 uF (220 or 470 would be better) to ensure compatibility with anything you might plug the preamp into. Polys in those values would be the size of a boxcar <g>, so a high quality "low impedance" aluminum electrolytic would be your best choice.

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Post by electropoet » Mon Oct 08, 2007 4:11 am

Thanks for all responses, it's good to have more opinions.
At the place where i should have those caps or transformers i soldered a short pieces of hard wire (like from resistors), so i can try tantals and polys myself.
As i have some nice 1:1 transformers i will also try them.

The thing is that i'm making a kind of mixer with mic pres (like API lunchbox) out of this modules, so it is expensive if all of them have good input and output transformers.

Brian:
I have some 600 Ohm gear, but i will not use it after this preamps, because the signal will go directly to the recorder. I'm building them exactly for this.

But as i have 6 Hamptones and 4 other preamps built for enclosure with place for only 8 modules, i could maybe sell two Hamptones (if Mr.Scott agrees) and buy output transformers for all of them.

I will report how i like polys, tantals and transformers.

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Post by digitaldrummer » Mon Oct 08, 2007 6:22 pm

and make sure those are polypropylene and not polyester film. You can also do the "bypass" trick on the large electro caps where you put a small (0.01uF) polypro in parallel with the electro.
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Post by brianroth » Thu Oct 11, 2007 12:14 am

Electropoet...I'm with you, as long as you PROMISE not to suddenly decide "I really need to drop this preamp's signal through my LA-2 (or clone) or Pultec EQ (or clone) before I print to the recorder...."

The undersized polycaps will bite you in the butt if there is any appreciable/desired LF signal coming from the preamp.

<g>

Hence, I tend to pig-out on coupling caps to account for those (unusual?) situations.

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