Marantz PM310 Upgrade Project

[kramtweeter]

Hello,
I'm embarking on a Marantz PM310 "upgrade" project. I really like the look of the front panel, so I thought it'd be great to preserve that beauty and replace the guts with some more modern components.

My initial goals are to:-

a) Improve the amplifier specifications. i.e. lower noise & distortion and increase power

b) Retain most of the front-panel features

The front panel assembly is quite modular, so some of these boards could actually be re-used without too much trouble. The main board I'd probably rip out and replace with separate amplifier modules and upgrade the power supply. I could design an entirely new main board. Or I could just design a small board for the input switching to retain those features and the mechanical switching.

If you have any suggestions, I'm all ears! What do you think is the best way to approach this project? Do you have any suggestions for amplifier modules or other key components? There is an internal height restriction of approx 65mm, so any components must fit within this limit.

Another thing I noticed on the schematic is that the tone control circuitry is in the feedback loop of the amplifier. This is kind of yucky as the wires have to go all over the place. Not good for low noise I'm sure. Let me know your thoughts.

 

[Karl vd Berg]

Go ahead! These are nice little amps.

Let me put the schematics of power amp (left channel shown), so suggestions can appear, and below the list of semiconductors...

[IMGDEAD]http://i64.tinypic.com/34r6cmd.jpg[/IMGDEAD]

Transistors (for both channels):
Q701 ~ Q704 > 2SA750
Q707 & Q708 > 2SC1400
Q709 & Q710 > 2S945
Q711 & Q712 > 2SC2240
Q721 & Q722 > 2SC945
Q723 & Q724 > 2SA733
Q725 & Q726 > 2SA2274
Q727 & Q728 > 2SA984
Q729 & Q730 > 2SD613
Q731 & Q732 > 2SB633
QN01 & QN02 > 2SC945
Q807 > 2SD571

Diodes:
Q705 ~ Q706 > 1S2471
Q713 ~ Q720 > 1S1555
Q805, Q811, QN03 > DS135D

 

[kramtweeter]

According to the spec, the amp has a THD of 0.3%, which isn't up to modern standards, so I wasn't planning to re-use any of the amplifier circuitry. Instead just rip it out and replace with some modern modules...

The input cross talk is poor - I can hear faint music even when another input is selected!

 

[Bigun]

More power probably means you need higher rail voltage so a new power transformer. Perhaps you don't need more power ?

I don't see any large heatsinks, perhaps a neat solution would be a Class D module. You can get some impressive modules on eBay these days, even with blue tooth connectivity as a bonus...

TPA3116 Bluetooth Amplifier Board Class D 50W 2 AMP Board CSR4 0 Module NEW | eBay

 

[kramtweeter]

Yes, more power isn't essential, but merely a nice to have... so you're right, I could keep the original transformer.

Interesting class D module, I hadn't realized you could get something like that with Bluetooth on it.

Only problem for this application is that I can't see any analog input? Also they don't give any distortion figures...

I do have an old Sony S-Master amplifier for parts with a pretty good SMPS in it. That might be suitable for a power supply for a class D amplifier module.

 

[kramtweeter]

Oh and the aluminum L bracket heatsink that goes right the way across the amplifier is bolted onto the chassis, so in effect, the entire case acts as a heatsink. So I suspect it can cope with more heat than might be apparent at first glance.

 

[kramtweeter]

Ok, so I've have some thoughts about how to do the tone controls. For best performance, I want all such controls to be right on the input board near the input sockets. Running sensitive analog lines to the front of the amp is a no-no. So it seems that a DC controlled tone control chip is called for here. Ideally it would run off split rails since it'd be nice to use the existing +/-15V.

Does anybody have any experience with such chips and could recommend a hifi quality one? The main criteria is low noise & distortion. I'm not keen on I2C control since then I'd have to add a microprocessor, but if I can't find anything good, then I'll have to consider those as well.

 

[kramtweeter]

The NJW1194 looks pretty nice for tone/volume control. Only trouble is nobody seems to stock them

http://www.njr.com/semicon/PDF/NJW1194_E.pdf

 

[kramtweeter]

Ok, here's the first revision of the block diagram. I've decided the best way to do the tone controls is digitally, plus I really wanted a digital input. These DSP's are cheap as chips, readily available and easy to solder.

 

[Bigun]

I'm not experienced with tone controls but I am lazy enough to know that if I were to try and do so much in one project, just because it looked easy on paper, I'd never get it done. I'd be doing it in stages so I could gain experience as I went, get some pleasure from the darn thing working at each stage so I didn't get bored before it was finished. So I"d sort out the power amplifier and get accustomed to the sound, give myself a 'direct' input to the power amplifier so I could bypass the rest of the existing preamp circuits and get familiar with how the amp sounds with a clean input. Then, only then, I'd tinker with the tone controls and try replacing those with something new, listening as I went to ensure I was getting the sound I want because theory is one thing but actual sound is something else.

There are a lot of Class D modules out there, if you don't need bluetooth just get one with an analogue input but likely you could find a point on the pcb to inject your analogue input. Posting over in the Class D forum might elicit some advice from those with more knowledge on these options.

You can also get ready-to-use tone control modules off eBay...re-building your Marantz with ready made modules is already a reasonable sized project. I don't have any experience with this module either, it's based on XR1075 chip but there are quite a few modules based on this chip available if you look around.

BBE Tone Adjust Module Bass Treble Volume Control 2 Channel FOR Hifi G6 008 | eBay


For the low prices of these modules you could order them and listen to them on the bench - little risk on your part, some fun in short order and you may have the answer to what you're looking for.

 

[kramtweeter]

Normally I wouldn't be interested in tone controls, but the front panel has them, so it would be nice for them to actually do something useful instead of being decorations.

Yes I know what you mean about being a lot of work, but I intend this project to be a quick turnaround one with rapid progress made. I don't have time to waste - this project is only a starter for something bigger and better

I can't see much useful data on the XR1075, anyhow I've decided to go digital, please see the block diagram. The main reason is that I don't want to run analog signals to the front panel and most of the analog chips are either hard to get or have poor distortion figures.

The amplifier modules are the least challenging part of the project - there are a lot of possible off-the-shelf options here...

 

[Bigun]

... I've decided to go digital, please see the block diagram. The main reason is that I don't want to run analog signals to the front panel.

I saw your block diagram but I've not done any digital design and debugging myself so it's outside my range of experience.

That module I linked to off ebay looks like it has analogue controls on it (the i.c. is analogue) - so you could take them off the pcb and mount them on the front panel with some wires back to the pcb.

 

[kramtweeter]

OK, after rejecting a lot of ideas and doing a lot of fiddling, I've come up with a mechanical layout that looks like it will do the business.

The beauty of this is that I can re-use much of the original main PCB, I just need to cut out the bits I want to retain. Also the mechanical controls, input sockets, speaker terminals and heatsink stay in the same places.

I may need to add extra fins and/or mass to the heatsink, but I'll worry about that bridge when I come to it. I've left extra room in the layout for this eventuality, so all should be well.

 

[wahab]

According to the spec, the amp has a THD of 0.3%, which isn't up to modern standards, so I wasn't planning to re-use any of the amplifier circuitry. Instead just rip it out and replace with some modern modules...

Distorsion is likely much lower than 0.3%, it s rather something like 0.03% as this amp has a double EF VAS, wich is not efficently maxed since the input stage is loaded with resistances, neverless that s a decent amp.

 

[jaycee]

If you're going to do tone controls, just stick to a conventional circuit using opamps. See Rod Elliot's preamp projects for a good example. He also has a good phono pre there. Ideal for this project.

 

[kramtweeter]

I've spent a productive day doing some mechanical work. I cut out the parts of the main PCB I want to keep and removed all the components that aren't being used. I was careful to leave all the tracks that I want on the existing boards. There's a lot more space inside now!

I managed to get my hands on a suitable transformer from a scrapped Cyrus AV Master 8.0 which is perfect for the job. Actually, it has one extra set of windings which I won't be using, the voltage is too high. I destroyed the original transformer to get the base plate which I needed to mount the new transformer.

I plan to keep the same circuit for the phono stage, just upgrade all the components. I'll replace all the resistors with metal film. Any suggestions for a pin-compatible low noise dual op-amp to replace the JRC 4558DD?

Next thing I need to do is get a bolt for the transformer and build a power supply for the amplifier modules. I have some 8200uF audio-grade capacitors from a scrapped Yamaha amplifier, but after removing them from board, I'm not so keen on using them as the capacitance is now measuring 7400uF on one and 7500uF on the other. Not so good...

The gold-plated PCB's for the LM3875 amplifier modules have arrived from audiosector.com, thanks man, super fast service to the UK. I need to order some more bits before I can assemble and mount these.

All in all, a pretty good day.

 

[Karl vd Berg]

...

I plan to keep the same circuit for the phono stage, just upgrade all the components. I'll replace all the resistors with metal film. Any suggestions for a pin-compatible low noise dual op-amp to replace the JRC 4558DD?

Next thing I need to do is get a bolt for the transformer and build a power supply for the amplifier modules. I have some 8200uF audio-grade capacitors from a scrapped Yamaha amplifier, but after removing them from board, I'm not so keen on using them as the capacitance is now measuring 7400uF on one and 7500uF on the other. Not so good...

...

For the phono stage you could also replace all those old caps with some of higher quality. There are not so many caps anyway. For the opamp, well, it's pretty hard to give any tips nowadays, so just choose any of very low distortion (OPA2134, LM4562 etc, etc, etc...). Actually you can buy a 8-pin socket and try as many opamps you wish as they don't cost too much.

For a new project I'd would use two fresh 10,000uF. These don't cost too much either.

 

[Welcome]

Gold plated PCBs? Man, these guys know nothing about electronics. The IPC gods specifically demand that any soldered surface must not contain any gold, due to the resulting poor solder quality and conductance (among other factors). Well, I guess DIY projects don't need to meet any industrial quality standards, and audiophoolery sells, so...

 

[wg_ski]

They must be expecting you to use AuSn solder. It's lead free and expensive, what more could anyone ask?

 

[kramtweeter]

Gold plated PCBs? Man, these guys know nothing about electronics. The IPC gods specifically demand that any soldered surface must not contain any gold, due to the resulting poor solder quality and conductance (among other factors). Well, I guess DIY projects don't need to meet any industrial quality standards, and audiophoolery sells, so...

Well why didn't you tell me earlier?

If I'd known that, I would have saved some money and bought the standard version of the board...

Lesson learned for the future.