Tape Repro Amp

[RSRecords]

Revisiting the idea of a tape repro amp with NAB. eq part is well documented for the most part. The major variations seem to come from the input section. Instead of going the discrete route ala MCI and some studers or the transformer route like Ampex. I was think of using a That corp 1200 Receiver. Other than the lack of gain, it seems like it could work with some resonant dampening. Would the head resonance dampening kill the benefits of having the high CMRR? Better off with a fet input like a opa2134?

 

[ruffrecords]

So much of the first amp requirements depend on the actual head that it is not really possible to answer your question, other than to say that for best noise performance you really do need some gain in the first stage.

Cheers

Ian

 

[RSRecords]

So much of the first amp requirements depend on the actual head that it is not really possible to answer your question, other than to say that for best noise performance you really do need some gain in the first stage.

Cheers

Ian

Ok, makes sense. Maybe is should look into a discrete FET front end. Maybe a good use for the new JFe2140? Hmm.. ok.

 

[ruffrecords]

Ok, makes sense. Maybe is should look into a discrete FET front end. Maybe a good use for the new JFe2140? Hmm.. ok.

If it is a high impedance head then yes a FET is the way to go, if not then there may be better options.

Cheers

Ian

 

[RSRecords]

So for most later pro machines like an MCI (jh110b/c with dcr ~4ohms and **L~5mH) there's probably a better option? Mci uses a lm394 as the preamp input. Should I be thinking more along the lines of a mic pre (plus filters)?

**edit: obviously L decreases with head wear.

 

[ruffrecords]

The impedance of replay heads increases with frequency due to their inductance. 5mH has an inductive reactance at 20KHz of 628 ohms so we are basically looking at a mic pre with bass boost for that one. I know a lot of pro phono preamps had a gain stage followed by EQ (as does the one you posted), so maybe for that head you could use a THAT mic pre followed by an EQ stage?

Cheers

Ian

 

[abbey road d enfer]

So for most later pro machines like an MCI (jh110b/c with dcr ~4ohms and **L~5mH) there's probably a better option? Mci uses a lm394 as the preamp input. Should I be thinking more along the lines of a mic pre (plus filters)?

With such a low impedance (100-200r in the critical 3k-5kHz range), you're in Cohen domain. The LM394 may be substituted for ZTX851.

**edit: obviously L decreases with head wear.

Does it so much?

 

[JohnRoberts]

Yup head amps are generally optimized for low source Z, like moving coil amps, or low Z mic preamps.

THAT corp makes mic preamp chips IIRC.

JR

 

[RSRecords]

With such a low impedance (100-200r in the critical 3k-5kHz range), you're in Cohen domain. The LM394 may be substituted for ZTX851.

Does it so much?

My understanding is that it can. Well Depending on the head probably. Ampex 350 8 track head is around 650mH new. I think they can drop to half of that before the gap is virtually exposed. Probably wouldn’t affect the preamp so much. Bass response extends as you weardown a repro head. Same with a cue head though recording gets worse because of issues with bias.

I’m going to order a couple that Corp chips and mess around.

 

[inkster]

I just had a 1/2" headstack for a 350 based 2 track put together by JRF and the PB head inductance was 500mH...cabling is very imporant as the cable capacitance has to be kept as low as possible otherwise the resonant frequency enters the audio range resulting in a crazy peak in response. The rule for high inductance heads is keep cabling short and use triax cable for anthing of length, mine were less than a meter.

 

[abbey road d enfer]

I just had a 1/2" headstack for a 350 based 2 track put together by JRF and the PB head inductance was 500mH...cabling is very imporant as the cable capacitance has to be kept as low as possible otherwise the resonant frequency enters the audio range resulting in a crazy peak in response. The rule for high inductance heads is keep cabling short and use triax cable for anthing of length, mine were less than a meter.

Another option is using a Virtual Earth input, as in Nagra 4..

 

[abbey road d enfer]

My understanding is that it can. Well Depending on the head probably.

Yes, wear causes the gap to increase slightly and the area to decrease, which both result in decreasing inductance. But I think it would require serious wear to produce a significant change that would alter the frequency response. It is clear though that, enlarging the gap results in progressive loss of HF.

Bass response extends as you weardown a repro head.

I haven't noticed that. Loss of HF is common, but LF response depnds dominantly on teh head_to_tape contact zone. I don't think that wear changes that significantly.

I’m going to order a couple that Corp chips and mess around.

Which ones?
The THAT mic preamps and multiple transistors are not as good in terms of noise as the 394 or the ZTX851.
The difference may be moot compared to tape noise...and the THAT mic preamps are quite convenient. You may even find existing ready-made PCB's.

 

[RSRecords]

So I think the 1512 could work here. I've seen them used for MC phono preamps with success. I'm a little turned around on the T-Bias of R4,5 and 6, the blocking caps, and head shunt resistors/caps. shunt caps and resistors are from the MCI schematic and the t bias is recommended from the 1512 datasheet. Repro head is ~5mH. plan is to set the gain in the 30-40dB and eq and some addition gain in the next stage.