Self-contained Tape Repro

Hi all

Unfortunately I have less and less time to log in the forum and contribute but as time permits I try to stay tuned to what all you felow DIYers do here.

In the last few month I did a lot of research into tape recording and reproduction in general and experimented lots of topologies and solutions.
I decided to build for my mastering needs an external tape rec/repro racks with direct connection to the heads of my two Studer A812s.
From the initial idea the whole thing went into two directions, a completely transparent capacitorless, jfet opamp based with DC servo circuit, and the second one a jfet/tube cascode circuit.
While experimenting with these two direction it came the idea to build as a challenge a self-contained battery powered tape repro electronics in the direction of the old Nagra and Stellavox portable recorders. The Stellavox portable machines were probably the most beautyful sounding recorders used in many audiophile classical recordings. Considering the small dimensions and the low voltage they were powered, their sound was a fantastic achievement.

So I had this idea to do it in a relatively small die cast box, 12Volt battery powered, class A, minimum number of capacitors in the direct signal path, IEC and NAB equalization and also manual tape EQ and head damping option with multiturn pots.
Slowly in some weeks the project evolved and managed to obtain some very good specs. Did a lot of tweaking and optimisation. Finally, with flux-loop measurement the freq response was linear from 100Hz to 150000Hz within 0.1dB, from 15000Hz to 20000Hz maximum deviation 0.2dB and between 25Hz and 100Hz 0.5dB. It is surprising that from 12V power I have a 10V peak-peak clean signal. Transistors were selected for minimum noise. Grounding and shielding done with great care considering the extreme gains necesary.

The final circuit has a very, very nice and clean sound (compared to the original A812 and the PR99).

Next time the no compromise external rack tape repro. I hope it will not take forever to finish it.

chrissugar

very nice!

I Agree 100%, THAT'S TIGHT!

Please excuse my ignorance but can someone please explain the use of this? Does it bypass the tape machine preamp and go straight to the recording heads?

Sorry for the stupid Noob question.

AFAI understand this box is for the repro heads, so for playback only.
Shortest, cleanest path from the repro (reproduction) head to line level, if I am not mistaken.

briomusic said:

AFAI understand this box is for the repro heads, so for playback only.
Shortest, cleanest path from the repro (reproduction) head to line level, if I am not mistaken.

Click to expand...

Exactly.
It is a clean path from the repro heads to the rest of the mastering chain (eq, comp, A/D converter) or to direct conversion to digital.
The great thing about manual EQ mode is that you can calibrate the thing with the multiturn pots very precisely, write down the values in a table and each time you have to play that specific tape you just dial in the right values.


chrissugar

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chrissugar

I love these machines! I'm getting a A807 very soon. Great project by the way!

Wowzers!  Where did you get those beautiful knobs that look like combination locks?

Wow.  Crazy.  Are the repro electronics in the Studer really that bad?  How much of a difference do you notice with your box? 

Audio samples would be fascinating to hear.

I attempted to do something similar with my Otari MX-55.  However my repro amp was a moddied Dyanco PAS3X.  The trouble I am having is tapping off the repro heard without having grounding/hum problems.  How did you manage to address that with your Studers? 

thanks,
Brad

substitute said:

Wowzers!  Where did you get those beautiful knobs that look like combination locks?

Click to expand...

The generic name for those is "turn counting dial" for multi-turn potentiometers.
After a  quick google I found some examples:
http://www.potentiometers.com/select_turn.cfm
and the potentimeters you use with these dials
http://www.potentiometers.com/select_multi.cfm
Unfortunately they are expensive, a dial 15-25euros depending on type and a potentiometer around the same price depending on type and manufacturer.

tommypiper said:

Wow.  Crazy.  Are the repro electronics in the Studer really that bad?  How much of a difference do you notice with your box? 

Audio samples would be fascinating to hear.

Click to expand...

The Studer electronics is not bad at all, it sounds very good. The only problem with these complicated computer controlled machines is that all the audio parameters are total recal. EQ, bias levels,etc are stored digital and to do this you need a lot of aditional circuits that will degrade in a way the audio. Also there are lots of old opamps in the signal path.
A simpe circuit with less utility circuits is a step cleaner.
When I will finish my other two repro circuits I will do a shootout with all the stock PR99MK3 and A812 electronics.

Brad McGowan said:

I attempted to do something similar with my Otari MX-55.  However my repro amp was a moddied Dyanco PAS3X.  The trouble I am having is tapping off the repro heard without having grounding/hum problems.  How did you manage to address that with your Studers? 

thanks,
Brad

Click to expand...

My preamp has no hum at all, no RF noise, only thermal noise reduced to minimum by selecting the transistors.

First, you have to be very careful with grounding. Power and signal ground, internal wiring etc. I religiously apply the correct way of connecting PIN1 to the metal chassis.
I recomend you read these docs:
http://www.rane.com/pdf/ranenotes/Grounding%20and%20Shielding%20Audio%20Devices.pdf
http://www.audiosystemsgroup.com/Pin_1_Revisited.pdf
http://www.audiosystemsgroup.com/Pin_1_Revisited_Part_2.pdf
It is important to use quality shielded cable from heads to the preamp and to leave the shortest possible wire unshielded at the heads. Use a separate wire to ground the complete metal head block to the preamp metal chassis if necessary.


chrissugar

Beautiful job, Chris... and it's good to see you again!

Hey Dave, I'm glad to see you  thank you for the nice words.
You will not believe it but a few days ago I was thinking about you and about Soundguy Dave. I had this nostalgia about the old forum times. I always thought it was a great loss for the forum when you both stopped posting. I loved very much your posts and I'm happy you are back.
Do you know anything about Soundguy? Is he well? any chance for him to participate again here?

chrissugar

Chris, Soundguy's studio is not very far from me, but I haven't seen or corresponded with him in quite a while. I've been meaning to drop him an email to catch up. As for me, I don't participate here to nearly the same extent as I used to, but I still drop by every now and then.

Absolutely beautiful work there Chris! I love it and that's proper DIY!

Dave's gonna love the new NYD-EQ I built. I'm putting a proper front panel on it next week. 4HE, 3 bands dual mono (+/-3dB), six frequencies per band. Sowter's biggest 4:1 -> EQ/attenuator network (IOAudio's nice Ni/Fe coils and Russian PIO caps) -> 7193+6SN7 amp. Been working with it for some 2-3 months now and it's so good that I just can't figure out anything to make it better.

Can't say I drop here much either. If you see Soundguy, say hello!

Thanks, Chris.  Looking forward to the shootout audio files.  

Scott Dorsey has mentioned he takes a direct out on his ATR-100, bypassing the audio i/o amp, so I've heard of this idea before.  I suppose for mastering it makes sense.

Just curious, why are you using the box approach, as opposed to a rack mount unit?  Why didn't you build something you can rack, or have on your mastering desk?  Do you need all those controls in the middle of the wire (rather than in your rack or on your desk)?

cheers

Thanks for the tips, Chris!  I'll check out those documents.  Would you be available to be hired to design some upgraded electronics for my MX55?

Brad

I just scored a Studer A807 and would really like to do something similar.  I'd love to see some pics of how you wired upto the heads.  How are you running those cables inside of the machine?

thanks,
Brad