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Davedog, audiokid Boswell


I just found out that this ADK preamp I'm working with works without any Input Transformer inserted...

I was recording a bass track - I had been switching up XFO's earlier this morning and must not have put one in after the last switch out...

The result is a very clean sound, can you tell me what's going on with this? I guess I was under the impression that an input transformer had to be inserted to pass signal - but I'm finding out that's not true.

Of course, there does need to be an OPAmp... but I was listening to this bass track - playing a Hofner, which can be a bit tough to nail down tonally - but when I listened to playback, it was very "clean" and much more transparent... I figured I'd "ping" you because I know you have the AP2...

But I'm more than happy to hear from anyone else, if they have an explanation as well..


DonnyThompson Sun, 03/22/2015 - 08:43

The mic does not work without an input transformer.

So, does this mean that the DI jack (1/4" instrument) is not routed through the XFO...? which would seem a little odd, because I've heard guys talking about how they've been plugging into it with bass guitar and getting all kinds of different tones with different XFO's, unless they are inputing through the XLR on the back (?? ) which doesn't really make sense...

Along with the 1/4" instrument jack on the front panel, there's also an impedance switch on the front, which is - and I'm quoting here - "for adding extra series resistance to very low-impedance vintage mics"... 1.5k ohms is what I'm finding...

Unless this pre basically defaults to a simple ( but pricey LOL) instrument direct box when the XFO is removed... ?

Boswell Sun, 03/22/2015 - 14:23

Unless they were true BS guitarists (believe me, I know the breed), it's likely that any experiments with transformer tone were done with an external DI box feeding the XLR mic input rather than using the DI input on the pre-amp.

It is an interesting concept, though. Because a guitar and external DI box is capable of putting out much more signal into an XLR input than even the highest output microphone, there could well have been an element of "what does this transformer sound like when we drive it into saturation?" They are not exactly the sort of trials you could do using a stomp box.

Davedog Mon, 03/23/2015 - 11:13

Donny send your questions to Larry Villella the founder and owner of ADK. As for your question to me...I'm not sure.....I think the DI on the AP pre doesn't use the transformer or perhaps doesn't need the transformer in order to work but the transformer may effect the signal chain if its in place using the DI. Make sense? I never thought much about it. I ALWAYS use the ADK Vintage N on bass.

Boswell Tue, 03/24/2015 - 03:09

I don't know the details of the ADK pre-amp design, but I would be very surprised if the XLR input were not routed through the transformer and then normalled at the DI jack. This would mean that inserting a plug into the DI jack would disconnect the transformer route, so when using the DI, it would make no difference whether a transformer were installed or not.

Davedog Tue, 03/24/2015 - 10:52

According to ADK, the designer tapped the DI AFTER the input transformer. So any differences I have ever heard have been when the op-amp is changed. I never thought much about this as I really only use the ADK Vintage N op-amp for bass in the DI. Since I always liked this op-amp with the Lundahl transformer there was never any reason to investigate. That channel hasn't changed in over 3 years. The second channel gets all the variations unless I need two channels of the same setup.

DonnyThompson Tue, 03/24/2015 - 20:22

Davedog, post: 426737, member: 4495 wrote: So any differences I have ever heard have been when the op-amp is changed.

Davedog Boswell

Well, it's not as if it's all in your head ...or mine, either for that matter LOL.

We are hearing character on the bass, via the OpAmp of choice, in your case, the Vint-Op -N, in mine, the John Hardy 990.

I wonder why the designer of the AP decided to tap the DI after the XFO? A noise issue maybe? And I wonder if this is something that could be easily modified?

Boswell Wed, 03/25/2015 - 03:20

DonnyThompson, post: 426755, member: 46114 wrote: I wonder why the designer of the AP decided to tap the DI after the XFO? A noise issue maybe? And I wonder if this is something that could be easily modified?

As Dave said, it's a matter of impedance. The XLR mic input is low impedance and the transformer takes it up to a medium impedance. The DI input has to be high impedance. This has all to be reconciled, and it usually means there is no perfect place to switch between the two sources. This is especially true when you have interchangeable op amps as the next stage, as they too will have differences at their own inputs.

In the upper-end pre-amps that I have been involved with at the design level (not this one, naturally), the insistence of the marketing departments that the unit have a DI input can cause headaches far beyond the simple working in of a switching jack socket. The simple alternative is to take the route that API and others did and compromise the DI input. This works fine for guitars that have active pre-amps but is not so good for piezo pickups.