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Author Topic: A Studer A810 playback head output mod  (Read 14731 times)

Offline docb

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A Studer A810 playback head output mod
« on: December 23, 2009, 04:15:02 PM »
We are working on an interesting Studer A810 mod right now. A client has sent us an A810 to mod for direct head output, and has ordered a Tube Repro to use with it. The nature of the head connection via very short cables to a small preamp on the head block makes this a bit different setup than the usual setup we find on other decks where the head cables are quite long and easy to reroute to jacks on the back of the deck. Splicing  long cables onto the short head cables is doable but seemed rather ungainly. So I decided to rethink this mod and see if we could squeeze in a 4PDT pushbutton switch that would allow us to offer switching between the internal playback electronics and a set of direct head outputs that will be mounted on a currently blank panel on the back of the deck next to the existing XLR jacks. After a few hours of fabricating some aluminum angles we think we have managed to squeeze the switch in where that scary red tape cutter button is normally mounted on the headblock (I guess some machines have this red cutter button and others have a black U shaped cover over that hole) . The mod requires no machining of the Studer and is completely reversible. We reasoned that we were offering additional safety by removing that cutter after imagining someone's 4 yo nephew wanting to press the pretty red button while uncle's Tape Project album was threaded up...

We're awaiting delivery of some nonmagnetic, very short screws to mount the switch bracket and should have a yea or nay about the utility of this mod when we get back to work after new year's.
« Last Edit: December 23, 2009, 04:20:23 PM by docb »
Dan "Doc B." Schmalle
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Offline ironbut

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Re: A Studer A810 playback head output mod
« Reply #1 on: December 23, 2009, 07:01:00 PM »
Killer news Doc!
I've been wondering when someone would offer an 810 for modification.

I can hardly wait to hear about any further developments and how the Repro does with it.
steve koto
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Offline docb

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Re: A Studer A810 playback head output mod
« Reply #2 on: December 23, 2009, 07:13:26 PM »
We listened to the stock playback electronics thru my HD600s and it was fine but neither Shawn nor I felt it was a particularly moving experience. It will be interesting to compare the stock Studer vs. Studer/Tube Repro vs. Nagra/Tube Repro in my system.
Dan "Doc B." Schmalle
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Offline oneobgyn

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Re: A Studer A810 playback head output mod
« Reply #3 on: December 24, 2009, 08:44:40 AM »
We listened to the stock playback electronics thru my HD600s and it was fine but neither Shawn nor I felt it was a particularly moving experience. It will be interesting to compare the stock Studer vs. Studer/Tube Repro vs. Nagra/Tube Repro in my system.


I would be interested in how it works out.

Dan you recall the fiasco when my first A810 was modded, specifically related to the short preamp cables and the need for a splice. Nothing but problems. There are many of us with Studers but don't want to do a mod that is permanent with potential downside
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Offline docb

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Re: A Studer A810 playback head output mod
« Reply #4 on: December 24, 2009, 09:18:27 AM »
Unfortunately I never saw the actual work that was done to your machine so I don't know exactly how it was executed. It's not impossible to just extend the head cables, but it would appear a bit inelegant to anyone who was looking inside the machine because the cable has to take a bend and go behind the head block plate right where the splice is made. The switch will create a mounting point that will make this more clean and stable, at least in theory. Now having been inside one I suspect that whatever was wrong with that particular mod could have been fixed pretty easily. Most likely it was simply an incomplete connection between the spring that Studer slips over the head wires as a rather unconventional form of shielding and whatever shield was added to that to cover the added cable.
Dan "Doc B." Schmalle
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Offline audioblazers

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Re: A Studer A810 playback head output mod
« Reply #5 on: December 24, 2009, 10:36:31 AM »
Doc, that would be the studer that I sent for modding. Surprise that this is the 1st studer sent for modding. Well at least I have the honour of beHi ing the 1st person to have his studer modded.
By the way what is the red tape cutter ?
Happy Listening

Song Soon Onn

Offline oneobgyn

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Re: A Studer A810 playback head output mod
« Reply #6 on: December 24, 2009, 10:44:22 AM »
Doc, that would be the studer that I sent for modding. Surprise that this is the 1st studer sent for modding. Well at least I have the honour of beHi ing the 1st person to have his studer modded.
By the way what is the red tape cutter ?

I wish I could say mine was first but several snafu's precluded successful completion.
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Offline Tubes n tapes

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Re: A Studer A810 playback head output mod
« Reply #7 on: December 24, 2009, 11:47:11 AM »
I have done a full tube repro on my A810 about two years ago. I did that same full tube repro mod on my Revox C270 six months before that.

That mod consists of a 23dB single tube flat gain stage built in the tape machine itself, connected to the heads by short cables.  The output of that stage is capable of driving a long cable so that the rest of the gain stages and the different EQs can sit in a separate external enclosure. The reason for that approach was mainly the fact that Studer heads don't do well with any significant capacitive loading, so the cables between the heads and the first gain stage need to be short. The output of the first gain stage has a fixed resistive output impedance and can easily drive a long cable without degrading performance.
The mod is permanent, but in order to keep the machine at least fully functional stand alone, the output of the tube gain stage also goes to a 23dB resistive attenuator and is then fed back to the original head preamp. That way I can take full benefit of the tube repro while the VU meters are still calibrated and the machine can still be used stand alone, without the tube repro.

The tube repro worked extremely well on the C270, so I thought: Let's do exactly the same thing on my A810. Only some small frequency curve corrections would have to be made in the gain stage to compensate for the difference in characteristics of the playback head in the A810 compared to the one in the C270. that way I could use the same back end amplification and EQs for both the C270 and the A810 without the need for adjustment.

The first gain stage in the C270 was placed behind the tape controls and counter. There is an unused slot that is reserved for a timecode card if I remember well. Unfortunately there was not such a nice location like that in the A810.  Since it is extremely busy inside the A810 there is only one spot with sufficient useful volume, which is on the left side of the middle towards the back. The only way to actually put something there is mounting it to the back plate of the machine. Inconvenient but feasible. That way the head wires could be kept under a feet. Making them longer will compromise the transparency of the sound.

So I built it up that way with the first gain stage built in an aluminum enclosure mounted to the back. After the success with the C270, my expectations of the A810 where very high. So the slap in the face was ever so more painful when I played the first tape. Somewhere between the hum and the noise there was also some music. I must have pulled the preamp out at least four times to double check the performance on the bench, which showed the same performance as the C270 preamp time after time.

To make a long story short, I found out the hard way why the head preamp of the A810 is sitting on the head block in a way that looks like an after thought and why there is so much shielding. The noise levels below the diecast chassis of the A810 are enormous. I could pick up volts over the ground lead of a shorted scope probe. The origin of some of that noise is the switch mode power supply for the 5V bus, but mostly the PWM regulated motor controllers. The designers made a well performing tape tension control, but clearly had no clue about EMI management.

I had to spend days to reduce the switching noise with at least 30dB to get the noise and hum of the preamp to a level that was sufficiently below the noise floor of the tape.

There is a happy end to the story in that the A810 now sounds better than anything that I have had the opportunity to listen to and that it has been my primary playback machine now for the last two years.

The moral of the story is: be very careful with running the un-amplified head signals through the machine. Long cables don?t work all to well with Studer heads, but if you want to do it anyway, my recommendation would be the run the head cables over the diecast chassis in the direction of the output plugs rather than under the diecast chassis.
Arian Jansen.

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VP of technology of the Los Angeles and Orange County Audio Society (LAOCAS).
ESL/OTL builder and modest Studer/ReVox collector.

Offline ironbut

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Re: A Studer A810 playback head output mod
« Reply #8 on: December 24, 2009, 12:04:00 PM »
Wow, that's quite a drama Arian! Thank god you know what you're doing! (I'd probably still be wondering what the heck was going on)

So, if you could offer more of your experience with us, I'd love to see a comparison of the different head spec's that Revox/Studer used in the different machines you've worked on.
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Offline mikel

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Re: A Studer A810 playback head output mod
« Reply #9 on: December 24, 2009, 09:51:12 PM »
Unfortunately I never saw the actual work that was done to your machine so I don't know exactly how it was executed. It's not impossible to just extend the head cables, but it would appear a bit inelegant to anyone who was looking inside the machine because the cable has to take a bend and go behind the head block plate right where the splice is made. The switch will create a mounting point that will make this more clean and stable, at least in theory. Now having been inside one I suspect that whatever was wrong with that particular mod could have been fixed pretty easily. Most likely it was simply an incomplete connection between the spring that Studer slips over the head wires as a rather unconventional form of shielding and whatever shield was added to that to cover the added cable.

a month or so ago i watched an 'expert' from this forum install a switch on the heads of my Studer A820 which allowed us to A/B the stock circuit and some custom electronics. he did an amazingly wonderful job of precision soldering of these tiny wires onto the switch; and then 6 hours later he put it back as original.

the head design i assume is quite similar to the A810.
we discussed the various choices of where a switch could be installed 'semi-permanently which would be elegant and not cause other problems. it sounds like you have come to the same conclusion that we did. using the hole for the fader as a place to install the switch.
as the Studers seem to be particularly solid machines mechanically i am happy to see that they are getting attention from you for the electronics. that can only lead to them being more optimized.




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Re: A Studer A810 playback head output mod
« Reply #10 on: December 28, 2009, 07:18:28 PM »
I think that all of this is a good thing, one way or the other!!! We will find out if like Doc said it is worth the fuss. I am one curious about the out come of this endevor!!!

Jay
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Offline docb

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Re: A Studer A810 playback head output mod
« Reply #11 on: January 04, 2010, 08:20:46 PM »
We got the preliminary cabling done today from the head to the switch, and from the switch to jacks on the top of the deck. We haven't connected the other switch output back to the internal preamp yet. The big pushbutton 4PDT switch was a very close fit, took a couple of tries to get it right. Shawn came up with a much better way to mount the switch than I had initially proposed, with just enough clearance around the capstan and other stuff under the head shield. Noise floor from the heads is well below that of the outboard preamp, so I don't think noise from any motor control electronics will be a problem. We did run the head cables up just behind the front cover rather than running them into the back of the machine. That was the only routing up to the jacks that seemed to be logical anyway.
« Last Edit: January 04, 2010, 08:32:15 PM by docb »
Dan "Doc B." Schmalle
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Offline jcmusic

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Re: A Studer A810 playback head output mod
« Reply #12 on: January 05, 2010, 09:20:36 AM »
We got the preliminary cabling done today from the head to the switch, and from the switch to jacks on the top of the deck. We haven't connected the other switch output back to the internal preamp yet. The big pushbutton 4PDT switch was a very close fit, took a couple of tries to get it right. Shawn came up with a much better way to mount the switch than I had initially proposed, with just enough clearance around the capstan and other stuff under the head shield. Noise floor from the heads is well below that of the outboard preamp, so I don't think noise from any motor control electronics will be a problem. We did run the head cables up just behind the front cover rather than running them into the back of the machine. That was the only routing up to the jacks that seemed to be logical anyway.
Hi Dan,
I realize and understand that you guys are just now trying to figure this thing out, would you mind posting some pic's of your work so far? I would love to see how you are going about this!!!

Jay
Redpoint Model D TT/Soundsmith Sotto Voce Cartridge Otari MX 5050 BII/BH Tube Repro deHavilland Model 222
 Ah Njoe Tjoeb 4000, Rogue Magnum 99, Korneff 45 SET Amp, Klipsch K-Horns Bass Bins/2" Tractrix Horns 2" BMS Drivers, Vintage Tubes.

Offline docb

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Re: A Studer A810 playback head output mod
« Reply #13 on: January 05, 2010, 10:36:04 AM »
It's a little crazy here today as we get ready to head out for CES. Maybe you can nudge me about it on Monday and we can get some pics posted then. Basically the switch is mounted to the headblock right above the capstan, on a bracket we fabricated from aluminum angles. The head wires come straight over to it and the shielded twisted pair cables to the output jacks are routed back through the hole that the head cables went through to the original preamp, and then up the front center of the deck right behind the front cover plate and over to the jack field at the top. The shields on the new cables are connected to the spring shields on the head cables - probably the most tricky aspect of the connection scheme.
Dan "Doc B." Schmalle
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Offline docb

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Re: A Studer A810 playback head output mod
« Reply #14 on: January 05, 2010, 07:58:59 PM »
Today's exploration was frequency response. We ran the A810 head into the Tube Repro that I use on my Nagra T. We had set the input impedance of the Repro to 90K ohms for the Nagra head, which gives us a slight rise at 10kHz and a slight rolloff at 15khz, a reasonable compromise for what might be a worn head issue, and not too far off what we usually use for the input load with Otaris and Technics.

However with the A810 (and IIRC we found this to be the case with an A80 head as well, a couple of years ago at VSAC) we sure don't have any HF rolloff issues with a 90K load! HF test tones into 90K ohms were rising, maybe 1.5dB hot at 10kHz and 3dB hot at 15kHz. I should mention that we have a bit more than a foot of shielded twisted pair cable inside the Studer and that was running into cheapo 1.5 meter STP cables to the Repro when we were seeing this 3dB rise at 15K. Perhaps you noticed other cable capacitance related issues besides treble rolloff that I have not spotted, Arian?

We will be installing trimpots to adjust the head damping to see if that can get the HF trimmed down a bit. That seemed to work for the A80, but if not for this A810 we can tweak the eq a bit too. In terms of relative levels we found the midband and bass output of the Studer head to be around 75% of the output level from an Otari head, which is one of the stronger playback heads. The Studer is about equal to the Flux Extended response head in terms of midband output level.

We looked at noise levels and waveforms next. I had a slightly microphonic EF86 in one channel that seemed to pick up a small 100kHz spike on that channel when the Studer was on, but the other EF86 didn't show any appreciable increase in it's noise floor whether the Studer was on or off, head connected or not connected. Pretty sure a tube swap will cure that slight spike and we should then be able to say the noise from the Studer is well down in the weeds.

Oh yeah, I suppose you guys want to know how it sounds. There is a very stable and smooth sense to the music, very smooth mids with a nice solid bass foundation. I would need to listen side by side to be sure but I'm guessing the Nagra might have a little more dynamic snap and the Studer might have a little more meat and potatoes. Won't comment about the highs until we get the HF response dialed in.
« Last Edit: January 05, 2010, 08:44:05 PM by docb »
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project