DIY Pre in new Tape Op

[Rodgre]

just finished the article on building a J-FET mic pre by Scott Hampton in the new issue, and I'm totally inspired to build my own high-quality 8-channel line mixer as a submixer for the 001. I don't hate the Mackie 8-bus as a means to submix the 001, but knowing that I could build a high-quality discrete circuit that will sound like I want it to...well, how can I not?

Now, where to get those transformers?

Roger

[NewYorkDave]

I don't get TapeOp anymore, but if you list the transformer specs, I can probably make a recommendation.

[leigh]

If I remember correctly, from a conversation with Scott, he has designed his own transformers and has them manufactured. According to the Hamptone site, the mic pre kits will be ready in a couple weeks. Once he's ready, I bet he'd sell you the transformers seperately, for a console project.

Of course, I'm not saying that something from Jensen wouldn't fit the bill as well.

Leigh

[rhythm ranch]

I don't get TapeOp anymore

???

Dave!

???

[NewYorkDave]

My sub ran out and there was that big controversy about information privacy regarding renewals, so I didn't bother. I get enough junk mail, spam and unsolicited commercial phone calls as it is.

[dgochenour]

You could get them here...
Jensen input transformers
Jensen output transformers

NewYorkDave--
the mic in was spec'd only as a 1:10, and the line out was spec'd as a 600/600 ohm 1:1. Any thoughts?

Don

[wing]

My sub ran out and there was that big controversy about information privacy regarding renewals, so I didn't bother. I get enough junk mail, spam and unsolicited commercial phone calls as it is.

no excuse... this is TAPEOP we're talkin about here!

[NewYorkDave]

I spoke too soon; my new issue was in the mail this evening. Maybe this is my last one... I dunno.

OEP (Oxford Electrical Products) and Beyer make some input trannies that would be quite suitable at much lower cost than the Jensen. I'll post more specific info later when I have some free time...

[loudmusic]

Totally not fair Dave.

I went ahead and signed up for all the spam-anation because I need the Mag that much... and no mag shows up!!

You must just be an all around nicer guy than me. karma or something?

Marty

[NewYorkDave]

OK, I've got a few free minutes, so here are some recommendations.

If you don't mind dealing with Newark's retarded website and waiting ten days or so for the parts to arrive from England, the OEP A262A3E should be suitable as an input transformer. Be sure to order the "optional" mu-metal shielding can, A262CAN, as well. Total cost for the transformer and the can is about $24. They're good transformers and at that price, they're worth the minor inconvenience of ordering. Please note that these are PC-mount.

http://www.newark.com/NewarkWebCommerce ... d=06WX2808

http://www.newark.com/NewarkWebCommerce ... d=06WX2809

For a stud-mount (chassis-mountable) solution, there's the Beyer TR/BV-351-310-005. They're cute little silver peanuts that make you go "awwww!" I bought ten of 'em earlier this year from Beyer's office in Long Island; I believe they cost me about $35 each.

At any rate, I wouldn't blow the bucks on a Jensen for what is essentially a "color" device (as Scott himself seems to be saying in his article). Save the Jensens for your more "transparent" projects.

A couple more suggestions:

I would add a small emitter resistor to the output Darlington, to provide some degree of short-circuit protection. This will increase the output impedance to a small degree, but it's cheap insurance. About 33 ohms should be OK. I notice the article doesn't give a figure for output impedance, anyway, so it might have been assumed that the user would always be driving modern-type "bridging" inputs and wouldn't really have to worry about it.

As a designer, I have some experience with the 2N5457 JFET and I can testify that the Idss (drain current) spread is ghastly. All JFETs are like that, but I recall that the 5457 was particularly loose in that respect. If I were building the circuit as shown, I would buy a fistful of 5457s so I could hand-select as necessary. Otherwise, I would provide a means of trimming the source resistor to compensate for sample-to-sample variation. It might be worthwhile to write to Scott and ask him to provide the voltages measured on his prototype so that you can adjust yours to be as close as possible. The schematic in the magazine doesn't show the source voltage; I'm assuming the drain voltage is held to a fixed value by the base of the Darlington--about 13.2 volts, I reckon, since it should be two diode drops above the Darlington's emitter voltage.

I built a lot of stuff with discrete JFETs back in da day and they're a lot of fun, but that sample-to-sample variation will drive you crazy if you let it. Good luck!

[scotthamptone]

Hello

We have our transformers made custom and would be happy to provide them to anyone interested. The price would be roughly $55 each. I won't know exactly, it will depend on how many we buy over the next 12 months, which will change dramatically with the release of the two new micpre kits. We will fill all micpre kit orders first, then we should be able to take care of any special orders.

RE 2N5457: The comment regarding the variation in JFET's is absolutely true. All the ones provided in our kits will be selected for both gain and noise, and we will probably give you an extra. I will also provide voltage bias information so you can check them yourself. The Drain in the circuit shown in the article runs slightly over 1/2 the 24 volt supply rail. One of the "1/2" symbols did not appear in the printed version so it may have seemed confusing.

The kits are coming soon, please stay tuned! Anyone interested in receiving updates, please email me and I will put you on the mailing list.

Peace
Scott@hamptone.com

[Elvis Debarge]

I spoke too soon; my new issue was in the mail this evening. Maybe this is my last one... I dunno.

OEP (Oxford Electrical Products) and Beyer make some input trannies that would be quite suitable at much lower cost than the Jensen. I'll post more specific info later when I have some free time...

Same thing happened to me... I didn't renew (not because of the information sharing so much as just out of forgetfulness and lack of ready internet access these days) and suddenly, this month I got a new issue. strange.

[lobstman]

OK, I've got a few free minutes, so here are some recommendations.

If you don't mind dealing with Newark's retarded website and waiting ten days or so for the parts to arrive from England, the OEP A262A3E should be suitable as an input transformer. Be sure to order the "optional" mu-metal shielding can, A262CAN, as well. Total cost for the transformer and the can is about $24. They're good transformers and at that price, they're worth the minor inconvenience of ordering. Please note that these are PC-mount.

Any suggestions for output trannies?