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Old 24th June 2014, 03:23 PM   #1
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Default Rane MA6S upgrade or replace

Hello All,

I've been very happy using a Rane MA6S amp to biamp my LCR speakers. Plenty of power and headroom. They're a little noisy, but not too bad. In an attempt to improve their performance, I replaced the input opamps with LME49713 and the noise was reduced a bit. My main question is, is it possible to bypass the input section and use as little of the amplifier circuitry as possible, with the intention of reducing noise and distortion by avoiding unnecessary opamps and capacitors. I'm using a highly modified Behringer DCX2496 as my crossover. The output section of my DCX2496 has been replaced with high quality film capacitors connected directly to the DAC output pins. Since there is no DC on the outputs (2.5Vp-p) is it possible to bypass the input opamps, high pass filter and opto/servo clip protection circuitry and connect the input directly to Q10 on the MA6S schematic? Or is the Z2 opamp (5534) required?

Any advice on moding is welcome. My other option is to build a 6 channel LM3886 based amplifier. Opinions are encouraged

Thanx
Chris Mitchell
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Old 24th June 2014, 04:03 PM   #2
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You could cut out the balanced input and low HZ filter going straight in on the volume control. That would work if you have enough analogue signal to drive the amplifier.
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Old 24th June 2014, 07:40 PM   #3
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Originally Posted by Flyingeyepro View Post
Or is the Z2 opamp (5534) required?
Yes, Z2 is necessary. The amp will not work without Z2.

If you wish to bypass everything, connect to R15, the input of Z2.

However, if your source has no volume control, you will have no choice but to connect to R12. Then upgrade C7 to BlackGates or something similar.

Regards
Mike
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Old 25th June 2014, 09:18 AM   #4
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Your noise is coming mostly from R13 and R14 once you've turned down input gain a bit (which I would expect to be doing - gain's a tad high at 33.5 dB and far more than needed when you're running anything close to a full nominal +4 dBu into it). Apparently those are used for dropping gain in BTL mode?

If you can live without that, remove the optocoupler Z3, jumper one of these resistors and replace the other by 220 ohms or so.

Note that the amp is giving away some practical input CMRR due to the choice of R2 and R4. I'd use something more like 100k (1% MF should be OK), which would increase common-mode input impedance to 50k.
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Old 25th June 2014, 01:06 PM   #5
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It must be my eyes but I'm not seeing any noise at R13 and R14.

Z3 is a Vactrol VTL5C2/2. This opto-coupler is a compressor. It does not increase gain. It is only activated when the output of the opamp Z2 hits a certain level. The compressor route is via R38.

There is a 2x gain when the input is driven in the balanced mode (R5,R6,R7,R8). I seriously doubt a 2x gain is the cause for noise unless the source is noisy to begin with.

The most likely cause is the gain setting at the feedback network. R24, R25 sets the amplifier's gain at 46x (33.4dB), which I feel is rather high. Unless one is experienced enough, I would not recommend lowering the gain as it could lead to instability.
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Old 25th June 2014, 10:49 PM   #6
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Originally Posted by Michael Chua View Post
It must be my eyes but I'm not seeing any noise at R13 and R14.
It's the resistors themselves. At 5k1 each, they make up the largest part of source impedance at the opamp input. Thermal noise voltage of 10k at 295 K in 20 kHz BW is 1.8 V. At the output, that's 83 V from these resistors alone, with volume turned down all the way. Including the 5534 and feedback network, we're at 100 V - by no means particularly noisy, but a value that decent integrated amps can reach as well, with an extra 10 dB of total gain.

Replace 2x 5k1 by 220R, and minimum noise floor drops to less than 40 V. If you have speakers of 100 dB SPL / 2.83 V / 1 m sensitivity, you can probably live with 3 dB of noise at 1 m anechoic easily...

(You can treat the amplifier as a big opamp and stuff part values into a noise calculator, with e_n and i_n as given for the 5534.)

As a nice extra, that should also improve distortion resulting from impedance unbalance at the inverting and noninverting opamp inputs. Stock, unbalance is 9k-11.5k, the modified version would improve this to about 1k8 or less, with a null at an input gain setting slightly below -20 dB or so.
(With .33R emitter resistors and I don't know how much quiescent current, I would expect practical distortion to be output stage limited though. 8 ohm speakers definitely preferred.)
Quote:
Originally Posted by Michael Chua View Post
Z3 is a Vactrol VTL5C2/2. This opto-coupler is a compressor. It does not increase gain. It is only activated when the output of the opamp Z2 hits a certain level. The compressor route is via R38.
Ah yes, that makes more sense. That's what I was suspecting initially, but I couldn't find a signal input.
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Originally Posted by Michael Chua View Post
There is a 2x gain when the input is driven in the balanced mode (R5,R6,R7,R8). I seriously doubt a 2x gain is the cause for noise unless the source is noisy to begin with.
Actually I wouldn't call that a 2x gain. Overall gain for both inverting and noninverting inputs is unity. Of course the contributions of both are added up, but both of them already counted in balanced signal levels as well. So if you send 0 dBu (balanced) into the input circuitry, you'll get 0 dBu (unbalanced) out.

I would expect the source plus input circuitry to be dominating noise in many cases when input level is cranked up all the way.
The combiner opamp already sees 10k total (btw, you can reduce R5-R8 values with a beefier Z1 - 4k7 should be easily doable with the '49713, but you need precision MF parts) - that's 1.8 V + x.
Consumer soundcard, 106 dB SNR @ 1 Vrms (no PGA): 5 V.
Ditto @ 2.2 Vrms: 11 V.
Semi-pro soundcard, 115 dB SNR @ 9 Vrms: 16 V.
So even in stock form, you may have to turn down input gain by 20 dB for source noise to drop below amplifier noise.

I therefore suggest that the OP do exactly that. Chances are that noise will be pretty much eliminated, while obtainable output levels still are more than sufficient. And it's free.

I've looked at the stock DCX output stage. It's not that bad actually. Granted, it does not take advantage of distortion cancellation in balanced inputs (only on the DAC side), and with all the output amplitude delivered by a single opamp, distortion could probably be a bit better, but the output is also adapted to unbalanced inputs easily. Bypassing it potentially allows high-frequency hash to wreak havoc in following amplifier stages if those are not up to snuff. But to each their own. At the very least I would leave the 121 ohm series resistors in (or install extra ones, precision MF) - capacitive load driving is not specified for the AK4393 outputs, and stability with more than a few kOhms of load may be marginal. (The datasheet is, unfortunately, rather tight-lipped when it comes to these things, which is frustrating. The quality of the output stages suggested also seems rather dubious, what with stupidly low feedback network impedance in Example 2. Apparently someone noticed later on, and the same circuit in the AK4396 datasheet looks a fair bit more reasonable. The properties of the DAC's analog sections might have changed as well, of course.)

Last edited by sgrossklass; 25th June 2014 at 10:57 PM.
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Old 25th June 2014, 11:17 PM   #7
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Quote:
Originally Posted by sgrossklass View Post
It's the resistors themselves.
Resistor noise is very minute. Noise to me is when you amplify a microphone signal 1000x from 1mV to 1V.

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Originally Posted by sgrossklass View Post
So if you send 0 dBu (balanced) into the input circuitry, you'll get 0 dBu (unbalanced) out.
If I send a 1V balanced signal into the input, there will be 2V at the unbalanced out.
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Old 25th June 2014, 11:45 PM   #8
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I appreciate the info provided. Since the cable length from my DCX2496 is <18", I'm confident that unbalanced is fine. Also, I don't need the HP filter, gain reduction vactrol or gain control potentiometer.

The output of my DCX2496 is my version of the passive film cap mod done by so many. I use 2 capacitors in parallel, a 10uf poly film and a .01 oil paper Sprague. Could I connect this directly to the input of R15 and Z2 pin 3?
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Old 26th June 2014, 12:49 AM   #9
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You can connect directly to R15 but a better way is to remove C7. This will disconnect the front end completely.

Then, solder a 1K 1% metal film resistor to R15. The other end of the 1K connects to your DCX2496 output caps. The 1K with 270pF (C31) will help to prevent RF from entering the input.
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Old 26th June 2014, 11:30 AM   #10
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If I send a 1V balanced signal into the input, there will be 2V at the unbalanced out.
You can sim it if you don't believe me.

But just to be sure that we agree on the definitions, 1 Vrms balanced is 2.83 Vpp between "hot" and "cold" (which may mean 1.41 Vpp to ground for both of them), and 1 Vrms unbalanced is 2.83 Vpp between signal and ground.

The inverting side has a gain of -(10k/10k) = -1. The noninverting side has a gain of 1 + (10k/10k) = 2, but the input signal is attenuated to half its original amplitude by a 10k/10k resistor divider, so overall gain is 1/2 * 2 = 1 there as well.
(In order to calculate these gains, always assume the other input remains tied to ground.)
If the input signal now is unbalanced at either input, overall gain obviously is unity, and anything in between is a linear superposition of these two extremes - including the case of minus half the signal on inverting and plus half the signal on noninverting.

You do often find a gain of 2 in balanced output stages, which may consist of a unity-gain buffer for the noninverting side and an inverting amplifier with a gain of -1 for the inverting side. If you send 1 Vrms unbalanced into one of these, you'll get 1 Vrms "hot" plus 1 Vrms "cold" = 2 Vrms balanced.


Quote:
Originally Posted by Flyingeyepro View Post
I appreciate the info provided. Since the cable length from my DCX2496 is <18", I'm confident that unbalanced is fine.
Doesn't mean you can't have a ground loop.

The inputs on this amp are always balanced and should prove some CMRR as-is, balancing source impedance merely gives much higher CMRR still. If I have my math right, 20k121 on one leg vs. 20k0 on the other will give a CMRR of about 44 dB, while even a worst-case 0.1% mismatch on the 10k resistors in the input stage would still give 54 dB.

(If you rewire the amp for direct unbalanced input, the CMRR you get is none - 0 dB. And minus the gain control, you wouldn't even have a way of attenuating the ground loop crap - or source noise, for that matter.)
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Originally Posted by Flyingeyepro View Post
Also, I don't need the HP filter, gain reduction vactrol or gain control potentiometer.
As I outlined above, if you truly didn't need the gain control you probably wouldn't have mentioned noise.

This "less is more" paradigm is stupid audiophoolery. It's quite definitely not universally true. Try building a 60 dB amplifier with negligible distortion and a bandwidth of at least 100 kHz or so using just one ordinary audio opamp. Good luck, you'll need it. The same with 3 stages in series - easy as pie.
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Originally Posted by Flyingeyepro View Post
The output of my DCX2496 is my version of the passive film cap mod done by so many. I use 2 capacitors in parallel, a 10uf poly film and a .01 oil paper Sprague.
The DAC provides a differential (balanced) output. The amplifier provides a balanced input. If I were looking at mods, I'd definitely want to exploit that, fancy caps or not. Actually you wouldn't even need caps then, as +2.5 V would be well within the MA-6S' input stage's common-mode range. (I assume both the amp and the DCX more or less agree about ground potential by means of a central safety earth connection.) The input stage is AC-coupled anyway.

BTW, you can hardly even measure the effect of generously-sized bipolar electrolytic coupling caps, let alone hear it.
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Originally Posted by Flyingeyepro View Post
Could I connect this directly to the input of R15 and Z2 pin 3?
You could. I hope I have made clear why I'd consider that a misimprovement though. Unless you enjoy shooting yourself in the foot, obviously.

Last edited by sgrossklass; 26th June 2014 at 11:40 AM.
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