DIY Marantz 7 based preamplifier

[haisaikat]

I recently got hold of a Marantz 7 Diy tube preamp kit available online with some additional aesthetic improvement made by the previous owner.

On the extreme left there is a power knob, next the 2 VU meters and then on right the volume knob and source selector knob.



Posting some internal pics





There are two transformers inside the cabinet own to power the main circuit and another to power the VU meters. The design has 3 input selector which is presently disconnected so that two sets of inputs at the rear are shorted and anyone being fed acts as input. The remaining one set of the 3 inputs are used as inputs to VU meter. The reason it's kept separate input is that the default intensity of line level inputs do not make them oscillate all the way so feeding speaker level inputs is the solution and hence the separate inputs. Although at present I am not using the meters. Also note that the dual outputs are shorted.

The device came with 1 rectifier tube and 2 X 12ax7 Haltron 7025 as drivers and 1 X 12au7 Brimar CV 4003 as cathode follower.

My initial listening experience is posted here

Pre-amp suggestions needed

After some period of silence its time to update on the progress I finally went ahead with Little Dot MK II with upgraded Mullard M8100 tubes. It was a gamble but owing to the less price of 11K I decided to give it a try. But I had one problem which is the LD had only one set of inputs and I...

www.hifivision.com

I need suggestions on this to see if there is further scope of improvement. I learnt online that in circuits like these the driver tubes make maximum impact to the output.

I was browsing through online options of 12ax7 that can be rolled here and read that long plate Version of Mullard 12ax7 are one of the best but the cost seems severely high and frankly the sources do not imbibe sense of trust. So the other option seems to buy the branded Chinese ones of the likes of Shuguang 12ax7 and it's variations termed PSVane.

Some folks have bashed M7 for its high noise and unwanted gain, not sure if I concur with that, yet.

Has anyone any other suggestions?

 

[sunilj]

Hi Saikat, do you have a schematic for this design ?

Found something similar (Music Angel), uses a different tube in the PRE stage - 6N4, which surprisingly enough is an equal to 12AX7. Any way to confirm ? I got this name off the rear view of your pics.

Schematic attached.

12AX7 link:

6N4 = ECC83 / 12AX7A NOS (square type getter)

6N4 = ECC83 / 12AX7A NOS (square type getter) the 6N4 was made in Peking till 1996 and has been used as stock tube from most guitar amp…

www.tubeampdoctor.com

 

[sunilj]

Can you post a closer pic of details from the rectifier tube ?

 

[haisaikat]

Hi Saikat, do you have a schematic for this design ?

Found something similar (Music Angel), uses a different tube in the PRE stage - 6N4, which surprisingly enough is an equal to 12AX7. Any way to confirm ? I got this name off the rear view of your pics.

Schematic attached.

12AX7 link:

6N4 = ECC83 / 12AX7A NOS (square type getter)

6N4 = ECC83 / 12AX7A NOS (square type getter) the 6N4 was made in Peking till 1996 and has been used as stock tube from most guitar amp…

www.tubeampdoctor.com

Cannot help praising your eyes .. I spoke to Nitin from whom I got this. It's indeed from Music Angel, per him based on his request they made a 12au7 and 12ax7 based version.

I am posting a close up of the circuit

Can you post a closer pic of details from the rectifier tube ?

I took out the rectifier inside and it's written 6Z4 on it. The exact version is

6Z4 Tube Full Wave Rectifier B7A 7-Pin base - Rectifier Tubeampdoctor

www.tubeampdoctor.com

I have also received from him an alternative rectifier tube Russian 6C4P-EV, not tried with it yet. The exact version is

6C4P-EV / EZ90 / 6Z31 / SV4005 tube (own box) - RECTIFIER tubes - Tubes-Store.com

tubes-store.com

 

[sunilj]

The schematic shows an LM317, possibly for heater supplies. Whats the TO3 (metal can) in your pic ? Closer pic of the text ?

 

[haisaikat]

The schematic shows an LM317, possibly for heater supplies. Whats the TO3 (metal can) in your pic ? Closer pic of the text ?

Here it is, part of the text is worn out

 

[sunilj]

Appears to be a 317 TO3 metal case.

 

[Hari Iyer]

Hi Saikat, do you have a schematic for this design ?

Found something similar (Music Angel), uses a different tube in the PRE stage - 6N4, which surprisingly enough is an equal to 12AX7. Any way to confirm ? I got this name off the rear view of your pics.

Schematic attached.

12AX7 link:

6N4 = ECC83 / 12AX7A NOS (square type getter)

6N4 = ECC83 / 12AX7A NOS (square type getter) the 6N4 was made in Peking till 1996 and has been used as stock tube from most guitar amp…

www.tubeampdoctor.com

If it was me, I would go and add a 0.1 uF/ 400v supply bypass capacitors in parallel to both the 10 uF capacitors feeding the tube plates.

 

[sunilj]

Not much scope with improving the 6Z4 rectifier. From the info published online, the Chinese version differs with the American equivalent (6X4), w.r.t pin layout. So, unless you are looking at a rewiring with a new rectifier, after understanding the voltages it's been designed with, your options are none.

The Pre and C.F are running nice tubes. I've tried and like both of them.

Your comment on gain and noise is a personal preference, unless it really bothers you.

So, what are you looking at improving. What are you comparing it with ? Have you tried another amp or speakers ? If your source has a volume control as well, its a good idea to keep that between 80-100% to get maximum benefit w.r.t S/N ratio, and operate your gig with the M7's pot.

 

[haisaikat]

If it was me, I would go and add a 0.1 uF/ 400v supply bypass capacitors in parallel to both the 10 uF capacitors feeding the tube plates.

So, what are you looking at improving. What are you comparing it with ? Have you tried another amp or speakers ? If your source has a volume control as well, its a good idea to keep that between 80-100% to get maximum benefit w.r.t S/N ratio, and operate your gig with the M7's pot.

I put this to both of you to summarize the comparison I have made so far based on my own observation, note that without tubes my system sounds bright to me nowadays. Red areas is where i need to improve, not sure if tube swapping can help

Area of Observation	M7 with above tubes	Little Dot / Mullard M8100 / 6H6Pi	Little Dot / Mullard M8161 / 6H6Pi
Bass	Best, very controlled yet punchy	Subdued	Good, huge improvement over M8100
Male Vocal / Jagjit	Best, very authoritative	Good	Good
Female Vocal / Shreya Ghoshal / Asha	Good but a bit brighter, need to be smoother	Best, very smooth, treat to listen	Good but a bit brighter
Treble Instruments / Highs	Good but a bit brighter, need to be smoother	A bit subdued, but can live with	Good but a bit brighter
Separation of Vocal with instruments when playing together	Separation seems hit, at times in few tracks feel like garbled, not in every song	Best	Good

 

[Rajiv]

I put this to both of you to summarize the comparison I have made so far based on my own observation, note that without tubes my system sounds bright to me nowadays. Red areas is where i need to improve, not sure if tube swapping can help

Area of Observation	M7 with above tubes	Little Dot / Mullard M8100 / 6H6Pi	Little Dot / Mullard M8161 / 6H6Pi
Bass	Best, very controlled yet punchy	Subdued	Good, huge improvement over M8100
Male Vocal / Jagjit	Best, very authoritative	Good	Good
Female Vocal / Shreya Ghoshal / Asha	Good but a bit brighter, need to be smoother	Best, very smooth, treat to listen	Good but a bit brighter
Treble Instruments / Highs	Good but a bit brighter, need to be smoother	A bit subdued, but can live with	Good but a bit brighter
Separation of Vocal with instruments when playing together	Separation seems hit, at times in few tracks feel like garbled, not in every song	Best	Good

Hi,

Replace the poly capacitors with PIO capacitors. Russian K40 capacitors are good and inexpensive. This should smoothen the sound .

Regards
Rajiv

 

[Hari Iyer]

Haisaikat,
What IC cables are you using and what wires for internal wiring of the preamplifiers?

IME, I used telephone cables for internal wiring of my line I/p and o/p. This I replaced with very thin tinned copper stranded wires. Both these wires attracted stray RF & EMI which got coupled with the audio and added a sparkle/ brightness to the sound stage. Using shielded cable was not an option as cable capacitance increases rolling of signal early.

To address the RF& EMI coupling I added an RC LPF at all inputs and outputs. This removed all the brightness and made the soundstage very mellow and smooth. You can consider this option

 

[sunilj]

I've found differences with PIO in some areas but not always. Good luck with this one.

I don't believe in the 'cable change' theory. Give it a shot, your ears may convince other wise.

 

[haisaikat]

What IC cables are you using and what wires for internal wiring of the preamplifiers?

I am using DAC ARX01A for all interconnects in my chain, and when i switch preamps to Little Dot just reuse the same ICs. Internal wires I am not sure of, can send pictures if that helps, i have kept what came by default.

To address the RF& EMI coupling I added an RC LPF at all inputs and outputs. This removed all the brightness and made the soundstage very mellow and smooth. You can consider this option

Thanks, will need some component suggestions, not that knowledgeable on this kind of work

Replace the poly capacitors with PIO capacitors. Russian K40 capacitors are good and inexpensive. This should smoothen the sound .

thanks, you mean the orange ones or some of the black ones. apologies for noob question

If your source has a volume control as well, its a good idea to keep that between 80-100% to get maximum benefit w.r.t S/N ratio, and operate your gig with the M7's po

This really helped, it goes without saying that I need a input selector with volume control in my chain, received @Aniket designed preamp, thanks to @sachu888 and @sadik for all the effort in putting it together, so my current chain is CDP->Aniket;s Preamp-->M7 --> poweramp. Turned off the digital volume control / preamp function of CD Player, kept Aniket's pre-amp volume at centre point which is unity gain position, note its damn transparent and does live up to its expectations, and then keeping the M7 volume at 9-10 o clock. The silence hum is reduced.

 

[sunilj]

A little confused here - why do you have two preamps in the chain ?

 

[Hari Iyer]

Haisaikat,
I am not aware about rest of your chain but shall mention what worked for me.

For all line inputs - cd player, tape deck and DACs I have used a 1 MHz LPF. This is at the input selector board. From the board to preamp input board I have used 350 KHz LPF.

In the power tube amplifier input just before the predriver tube grid I have added one more LPF @ 700 KHz.

Hope this helps.

 

[haisaikat]

A little confused here - why do you have two preamps in the chain ?

I understand your concern, this was previously discussed here in below thread https://www.diyaudio.com/forums/tubes-valves/247232-music-angel-pre-amplifier-modifications.html

Basically I needed a input switch with volume control option in the chain that I had earlier in integrated setup.

 

[sunilj]

THAT is one crazy thread, considering the number of different tube types the same design has been modded for.

Two preamps piggy-back, for whatever reason is not a bright idea. You have two pots, two different gains, two preamp stages and two output stages. Have you tried with the DAC direct to the M7 ? Whats the drawback here ? Losing an additional input ?

That thread suggests various mods, maybe try those.

Just reread your original post, you've got a single input and the remaining have been butchered for various reasons.

You should try that single input and let us know how it sounds. Can't be that bad.

 

[haisaikat]

THAT is one crazy thread, considering the number of different tube types the same design has been modded for.

My apologies, probably copy paste error, I tried to refer to this thread.

Pre-amp suggestions needed

For the last 4 years I was happily using a CA 650 A integrated amp with MS Aviano 6 FS and was quite happy with source being RPI with Rune / Moode fed via ODAC (objective DAC). This year I made three upgrades, introduced CA 851C CDP which acts not only as transport but also as DAC which is...

www.hifivision.com

Have you tried with the DAC direct to the M7 ? Whats the drawback here ? Losing an additional input ?

Yes, its exactly similar, I know there is a longer signal path here but to my years with only M7 it's still the same. Regarding multiple pots, it's the same reason why I had all of them maxed initially and controlling via the digital volume control of source.

 

[sunilj]

Don't know how to help further. You've got good tubes already, can't say any more 'rolling' will change the sound appreciably.

Maybe give the mod suggestions mentioned by our members. Just a thought, have you tried a different and /or speakers to see if they interact differently ?