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Old 29th March 2011, 03:33 AM   #21
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The Mellotron has only one channel, but there is a tape head for each key and there are about 2 1/2 octaves- about 30 tape heads, all in series. So any head has to make voltage across the entire head array. Naturally it is going to be noisy. The tape equalizer uses bipolar transistors- with a fairly low input impedance as these things go (think: late-1960s consumer electronics design). I suspect that while it can be made a little quieter by replacing the ancient transistors in the circuit, that with a little redesign that it might be made a little quieter yet- perhaps with an FET input? Its obvious that the EQ amp does not have a lot of gain- it appears to only have two gain stages. The real issue is I have no idea what they set up for EQ. At this point it seems that I'll just have to take it apart and trace it out. Once I have done so I will post the schematic. That will happen *after* this Thursday- I have a gig with it Thursday night.
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Old 29th March 2011, 04:17 PM   #22
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I'm assuming that any good NAB equalizer preamp would be good.
As they used the same technology that any tape machine uses.
I read that john lennon would record his own sample tapes on an open reel deck at the studio and install them in his mellotron at home.
And that an adapter was eventualy created with guides for 1/4" tape.

The reason I had asked if you had tried replacing the tapes (before I realized that there was'nt a record function) is that back 1990 3m came out with the 996 tape and was virtualy noiseless.
It was what saved the analog recording industery until the 24 bit format became available.
The formula was sold to BASF and now I don't what brand it is under now.
That was when ampex came out with the 499 formula which is comparable but I prefer the 996 as it was better except for some small clicks inherent in the tape problably due to improper mill grinding.
They could only be barley heard in the blank passages they could not be erased and disappeared with the DBX on.
I no longer need to use the DBX function at all on my MSR-16 except to increase the dynamic range and even that is marginaly useful.
as it is much cleaner sounding without it.

I first started using 996 on my FOSTEX R8 1/4" 8-track and what world of difference it was compared to the ampex 457 and 456 that everybody swore was the best.

I still have a few reels a 226 that they used way back then and for the life of me I still don't know how they got by with it.

Besides my tascams I also have two OTARI's the MTR12 and MTR15.

I will compile the plaback headamp circuits to send them to you and then you could go from there. jer
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Old 29th March 2011, 04:36 PM   #23
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Quote:
Originally Posted by geraldfryjr View Post
I'm assuming that any good NAB equalizer preamp would be good.
As they used the same technology that any tape machine uses.

The reason I had asked if you had tried replacing the tapes (before I realized that there was'nt a record function) is that back 1990 3m came out with the 996 tape and was virtualy noiseless.
It was what saved the analog recording industery until the 24 bit format became available.
jer
The tape formulation in the Mellotron is not much of an issue. The noise that concerns me has nothing to do with the noise of the tape- it is everything about the noise made when the machine is just sitting. Its before the volume control, but I don't have to play a single note and the noise if the EQ preamp is quite evident.

Quite often the input impedance of a preamp and the source impedance can have an interaction that results in the unit making more noise than it has to. In this case, I suspect that part of the problem is that the input impedance is rather low while the combined series impedance of the heads is rather high. That is why I have been thinking of an FET input- if I could create a high input impedance it might be that a lot of the preamp noise will vanish.
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Old 29th March 2011, 04:38 PM   #24
tomchr is online now tomchr  United States
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Quote:
Originally Posted by atmasphere View Post
The hiss is entirely from the EQ circuit, and I suspect that the transistors have become noisy. An opamp circuit is not out of the question, but I have to sort out the EQ first- that is why a schematic would be handy.
I highly doubt that the transistors have become noisy. I suspect the hiss is by design, so to speak. The tape heads output a fairly low signal and I think they're fairly high impedance as well. That's about the worst case for noise. Combine that with the fact that you have a lot of channels running in parallel being summed at the end would make matters worse.

I would vote for reducing the noise bandwidth if you can. But that would probably require a design change or tweak.

~Tom
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Old 29th March 2011, 05:13 PM   #25
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I understand, just thought I'd share some of my experiences.
Cheers! jer
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Old 29th March 2011, 08:15 PM   #26
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Quote:
Originally Posted by atmasphere View Post
Quite often the input impedance of a preamp and the source impedance can have an interaction that results in the unit making more noise than it has to. In this case, I suspect that part of the problem is that the input impedance is rather low while the combined series impedance of the heads is rather high. That is why I have been thinking of an FET input- if I could create a high input impedance it might be that a lot of the preamp noise will vanish.
I would suggest the opposite, the heads are likely to be fairly low impedance, and the preamp designed accordingly - but in any case, increasing the input impedance of the preamp it more likely to increase noise than reduce it.

You say the noise is there all the time, have you tried shorting all the heads out (same as shorting the only head in a normal tape player), what does that do to the noise?.
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Old 29th March 2011, 09:15 PM   #27
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Originally Posted by Nigel Goodwin View Post
I would suggest the opposite, the heads are likely to be fairly low impedance, and the preamp designed accordingly - but in any case, increasing the input impedance of the preamp it more likely to increase noise than reduce it.

You say the noise is there all the time, have you tried shorting all the heads out (same as shorting the only head in a normal tape player), what does that do to the noise?.
Hmm. The heads are typical of 1/2 track mono heads designed for 1/4 inch tape. They are on a special track that can be shifted so you can pick up the 3 different tracks on the 3/8" wide tape. Since there are about 30 heads in series, even if they had a fairly low impedance (for a late 60s design), it seems to me the resulting impedance will still be quite high.

I've not tried shorting the input yet. That seems like it would tell us a lot about what is going on.
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Old 29th March 2011, 09:39 PM   #28
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So there is only one preamp and the tape head combines the signal from the differently pitched tapes? There might be an easy way to mute the entire circuit till a key is hit. As far as sound quality, if that tape is more than 10 to 15 years old and the thing was used (this tape gets used a lot more than normal tape machine usage (how many times has the middle C been used, 1000's ? )) there may not be any "quality" left on that tape. Also, when was the last time tape head was aligned and the repro electronics calibrated? (although Im not sure how you would do this without an alignment tape or I guess pieces of tape?)
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Old 29th March 2011, 10:28 PM   #29
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I rebuilt a good portion of the instrument when I obtained it about 7 years ago. The tapes are new. However, its not a problem when a note is being played (although I suspect that if there was less noise in the system, it would sound better).

The issue is, when no tape is being played, the machine makes a lot of noise and its all before the volume control.
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Old 30th March 2011, 08:51 AM   #30
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When you get a chance try to post the transistor number and I will see if I can find a spec sheet on it.
Some a have high NF (noise factor)rating of like 6db to 10 db or more.
And some of todays transistors are tipicaly 3db or less.

I remember in my days of repair and tearing apart 8-track tape players(as I was obsessed with the idea of building my own super quality stereo system from scratch) that some head preamps were quite noisey and some were super quiet.

I noticed that the noisey ones used (A) type transistors and the less noisey ones used (B) type transistors and the preamp boards were all pretty much the same design style and construction using 2 transistors per channel.
I found this to be true when I had acsess to a ton of scematics and various stereo units that I worked on that came through the T.V. repair shop where I was working in the early 80's.
And then I discoverd opamps (in the late 70's).

FYI,For those whom just starting to follow this thread the mellotron's noise rating was -55db which was is a bit high even for the early to mid 70's. jer

Last edited by geraldfryjr; 30th March 2011 at 08:59 AM.
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