Marantz 7 clone tape head preamp

[Foxbat]

Yes, and it is well described in the video linked here. Also in the video you will find my description of its sound, which is very, very good.

Now, about the comparisons... I think the A807 is a very easy target and that preamp will - in my view, of course - beat it.

I can't speak of the M21 and I have no experience with it, I only have the M15A.

tapepath - you are very welcome, I am always willing to share my experience. Your construction is indeed very nice.

You are right about the input loading - I also put the trimpot in that position, but you need to be careful with it, as it and the HF EQ pot will both affect that region, so I would start with that 100k pot wide open while adjusting the EQ, while watching for some high frequency peaks - sometimes you will see 20KHz or 25KHz ones... then you can tame them with that trimpot.

Another minor comment has to do with the front panel mounted pots. I have done that in some of my preamps. It is, of course, very convenient, but you always run some additional risk of power line interference due to their long wires. Twisting the wires tightly will reduce that pickup, but you will still be able to see those peaks on the spectrum analyzer. They are very small, and nearly impossible to hear, but they are there.

That is why I eventually moved to mounting the adjustment as close as possible to the EQ network. Not as convenient, but less prone to pickup.

I am still curious about your project as I don't see any switching there. How do you select the EQ you want?

 

[tapepath]

Thank you for the very helpful recommendations. I tightly twisted the connecting wires in a drill and then inserted them in a grounded shield braiding. I considered placing all of the wiring inside copper tubing, but will want to do some additional testing before I try that approach. For me, this project is a working test bench, a way to see how the different calibration adjustments effect each other and how they sound in comparison to a reference. I designed the enclosure so each panel is removable to give easy access to any part of the construction.

 

[tapepath]

 

[Foxbat]

So it looks like you are not switching between the different EQ's, but adjusting as you need. I then presume you are going to use the ten-turn dial on each one, so you could quickly return to the desired EQ?

Waiting to hear your impressions!

 

[tapepath]

At this point I'm concentrating on an NAB 7.5 response for this project. I enjoy searching out those vintage London and RCA tapes made in the late '50's and early '60's that have some amazing sound qualities. My principal listening test tape, for this project, is the 1958 London recording of Britten's Peter Grimes. The recording technique they used involved placing the chorus and singers on a stage that had been marked with numbered squares. Each performer was positioned in a specific location after testing how it related to the stationary microphone "tree". Then as the scene in the opera required the singer would move to another marked area of the stage. So if, for example, the Peter Grimes character moves from stage left to the rear of stage right he would continue his dialogue and walk to the assigned area. The result is a palpable recreation of a sound palate with the chorus, singers and orchestra. While I'm sure this was done to highlight the then new "miracle" of stereo sound it meant that spatial clues could be preserved.

 

[adrianywu]

Yes, and it is well described in the video linked here. Also in the video you will find my description of its sound, which is very, very good.

Now, about the comparisons... I think the A807 is a very easy target and that preamp will - in my view, of course - beat it.

I can't speak of the M21 and I have no experience with it, I only have the M15A.

tapepath - you are very welcome, I am always willing to share my experience. Your construction is indeed very nice.

You are right about the input loading - I also put the trimpot in that position, but you need to be careful with it, as it and the HF EQ pot will both affect that region, so I would start with that 100k pot wide open while adjusting the EQ, while watching for some high frequency peaks - sometimes you will see 20KHz or 25KHz ones... then you can tame them with that trimpot.

Another minor comment has to do with the front panel mounted pots. I have done that in some of my preamps. It is, of course, very convenient, but you always run some additional risk of power line interference due to their long wires. Twisting the wires tightly will reduce that pickup, but you will still be able to see those peaks on the spectrum analyzer. They are very small, and nearly impossible to hear, but they are there.

That is why I eventually moved to mounting the adjustment as close as possible to the EQ network. Not as convenient, but less prone to pickup.

I am still curious about your project as I don't see any switching there. How do you select the EQ you want?

I agree totally. I came to the same conclusion while working on my balanced tape head preamp. I ended up soldering the components as close as possible to the main circuit and use relays to do Eq switching. For the Marantz 7 clone, since I am only using it with 4-track tapes, the Eq components are soldered on the board. My balanced pre is used with my Nagra T, which has a high inductance head. The input is loaded with 500K (250K each phase) with 50pF of capacitance to achieve a pretty flat FR with only -1.5dB at 20kHz. The Marantz 7 managed an almost perfect FR with the Revox B77 without changing the original input loading resistors that came with the board, with only -1.0 dB drop at 20KHz. Pretty impressive for a consumer machine. However, when I tested the Nagra with the Marantz clone, the FR started dropping at around 12kHz and was down about 12dB at 20kHz.

 

[Foxbat]

Of course, as the Nagra heads have very high inductance. So if it is 700mH (from memory) and your input resistor is 47K you get 11KHz the -3dB point. 500K is not really needed, but something like 150K would be great.

 

[tapepath]

A device that I found helpful is an inverse NAB network offered by KAB Electro Acoustics. It allowed me to connect a signal generator (works best with 50 ohms output impedance) to the NAB network and look at the frequency response. Granted, it doesn't substitute for a calibration tape or a flux loop, but it can give you a beginning point at seeing the overall compliance with the NAB EQ.

 

[GregoryC]

Tapepath
Can you write which elements you removed from the PCB? Which ones you installed for NAB correction?

 

[tapepath]

1). Remove R13 and C9.
2). Replace R12 with 10k trim potentiometer.
3). R4 is paralleled with 360 ohm resistor.
4). C2 is replaced with 0.22uF capacitor.
5). R8 is removed and replaced with 470k ohm resistor.
Optional suggestions:
1). Replace R1 with 150k trim potentiometer.
2). Replace R4 with 1k trim potentiometer for adjustable gain and channel balance control.
3). Double the value of all of the power supply filter capacitors (however they are glued in place and are not easy to get off of the board).

I hope this is helpful.

 

[Foxbat]

Looks good. The only difference - since I was shooting for IEC, I completely removed the R8 resistor. I usually don't like leaving such circuit completely open, so I initially put a 1.5 Meg there, but someone had suggested removing it and it worked well, but I would advise some caution there.

One needs to be careful replacing the R4 with a 1K pot, as the adjustment might become very touchy, hard to get the exact point. I usually limit the effect by putting a fixed resistor in series with a trimpot. Perhaps 390 Ohm with a 200 Ohm trimpot.

 

[GregoryC]

tapepath
Where or how can I purchase PCB for Elliott Sound Products?

 

[GregoryC]

Thank you very much for your valuable advice. I have already ordered the PCB

 

[tapepath]

Good luck on your project! Let us know your progress!

 

[Kcin]

This is all very interesting!

I had not seen this project before.

Thank you @Foxbat for sharing your knowledge with the community. Perhaps some clarification if you see this:

You mention in your video you change the 510R resistor in the first stage cathode with a 360R in || for altering the gain ---- @ tapepath mentions it here as well:

Marantz 7 clone tape head preamp

Yes, and it is well described in the video linked here. Also in the video you will find my description of its sound, which is very, very good. Now, about the comparisons... I think the A807 is a very easy target and that preamp will - in my view, of course - beat it. I can't speak of the M21...

www.whatsbestforum.com

Its not clear if you actually implemented this mod or not in your final iteration - it. looks like @tapepath has implemented it via his pots .Is this something that works well with moving the pole by elimination of the 630k resistor as well?

I am going to collect the parts to build this and see where it takes me- thanks again. Right now I use a deHavilland-- @tapepath you mention you can offer comments on the sound of the M7 clone vs the deHavilland?-- what are your impressions?

thanks for any help!

 

[tapepath]

I'm afraid I haven't done a meaningful comparison between the two units. The 222 is combined with a Tandberg TD20A for 4 track playback and it sounds excellent. It also works well with a Dolby unit for Barclay Crocker listening.
I felt it was time to get a better understanding of the physics and mathematics relating to tape playback, so the M7 has become a test platform for me. It is a good way to learn about modeling the various interactions between the playback head and the circuitry and then make measurements to verify the results. With the unit's adjustment capability and easy access for modification it is a great learning tool.

 

[Foxbat]

I still have that HP distortion meter as well... it is truly amazing how reliable these are!

 

[hannemanpl]

Hi, everyone!

I came across this post because I am looking for an inexpensive tube amplifier for my Revox B77 to see how it will sound in this configuration.



I carefully read the posts and watched @Foxbat videos. Great job—very informative and to the point. To be on the safe side, however, I drew up a diagram with changes for IEC 15 IPS correction. I marked the components that need to be removed and replaced. As you suggested, I also marked the replacement of the capacitors in the power supply with higher capacity ones. I also marked the filtration in the 12AX7 tube power supply with a green question mark. Should these capacitors also be replaced with higher capacity ones?



Are there any other components that you would replace or change after prolonged use of the preamp? If so, please share your tips.



I ordered a PCB and will be building my own unit. As soon as I get it up and running, I will of course share photos and my impressions.



Best regards!

 

[adrianywu]

Hi, everyone!

I came across this post because I am looking for an inexpensive tube amplifier for my Revox B77 to see how it will sound in this configuration.



I carefully read the posts and watched @Foxbat videos. Great job—very informative and to the point. To be on the safe side, however, I drew up a diagram with changes for IEC 15 IPS correction. I marked the components that need to be removed and replaced. As you suggested, I also marked the replacement of the capacitors in the power supply with higher capacity ones. I also marked the filtration in the 12AX7 tube power supply with a green question mark. Should these capacitors also be replaced with higher capacity ones?



Are there any other components that you would replace or change after prolonged use of the preamp? If so, please share your tips.



I ordered a PCB and will be building my own unit. As soon as I get it up and running, I will of course share photos and my impressions.



Best regards!

I would suggest you built it as is and listen to it first so that you have any idea how it sounds. Otherwise, you will not know whether the modifications are improvements or not. Also, tube rectifiers have a limit on how much filter capacitance they can work with, and so increasing the filter capacitance does not necessarily make it better. I built mine with the original parts except for the Eq being modified for NAB. It also has very good performance, better than the internal repro of most tape machines.

 

[Foxbat]

I see you decided not to reduce the 510 Ohm resistor - without doing this you might find that your gain is a bit too low.