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No regulator for best sound?
No regulator for best sound?
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Old 24th October 2017, 05:08 AM   #1
hollowman is offline hollowman  United States
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Default No regulator for best sound?

I've posted several times on a late-1980s Magnavox (Philips) CD player. Due to it' simple, accessible layout, it's an easy "breadboard" project.

The stock unit's PSU contains only one main voltage regulator: an LM7805. Its 5V gets routed to the SAA7210, TDA154x, and a few other ICs.

What's surprising is that the analog section (I/V and LPF opamps, etc.) are fed unregulated power! E.g., +/- 15v. Indeed, there is over 50mV of AC ripple on the +/- 15v opamp power pins. Compared to less than 2mV on the +5v DAC.

As part of my experiments with this unit, I am no longer using the stock I/V and LPF opamps ...Inside this CD player (see photo) I am currently feeding the DAC to a discrete I/V output stage. The discrete I/V is designed to work on 12-18vdc (supposed to sound best at 18vdc).

So I fed the discrete I/V the same unregulated +15v that orig fed the opamps. The sound quality is very good.

Because the discrete I/V can run on 12v, I decided to regulate the stock unregulated +15v with an LM317 circuit (the LM317 was config'd to output 12v). While the ripple reduced several orders in magnitude, the sound quality worsened: bass became puny and dynamics became very weak.

Returning to stock unregulated +15v, performance returned to orig.

Any guesses as to the why the sound worsened despite improve ripple performance? E.g., lower voltage (power) for I/V, etc.

Note: This CD player's stock transformer can only offer a max of 15vdc. So that's the max voltage I can afford for any project power.

Below: Stock PCB of Magnavox CD 2000
Click the image to open in full size.

Below: Same Magnavox CD 2000 as above, but using Rudolf Broertjes' I/V
(Note: the I/V is simply powered by the opamp socket 15v power pin)
Click the image to open in full size.
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Last edited by hollowman; 24th October 2017 at 05:18 AM.
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Old 24th October 2017, 05:24 AM   #2
anatech is online now anatech  Canada
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No regulator for best sound?
Hi hollowman,
I rather suspect that it was the lower B+. Too bad Philips made such cheap ... stuff. I serviced those machines and am well aware of how many corners were cut to save a little $$.

For all the effort you are putting into that machine, consider installing a new transformer that supplies higher voltage and regulate that down to the same levels (minus the ripple). If the regulators get hot, consider using an inexpensive switching regulator for the digital chips (the 5V supply).

Basically, eliminating the ripple and noise can only lower the noise floor and distortion from outside effects. There is no way running your analogue sections off a supply with ripple and noise will ever be better than a nice quiet power supply.

-Chris
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Old 24th October 2017, 10:48 AM   #3
DF96 is offline DF96  England
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Quote:
Originally Posted by hollowman
Because the discrete I/V can run on 12v, I decided to regulate the stock unregulated +15v with an LM317 circuit (the LM317 was config'd to output 12v). While the ripple reduced several orders in magnitude, the sound quality worsened: bass became puny and dynamics became very weak.
What measurements did you do to determine which sound was closest to the original sound heard in the studio? All you have expressed is a preference, which tells us not very much.
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Old 24th October 2017, 04:21 PM   #4
Chris888 is offline Chris888  United Kingdom
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Quote:
Originally Posted by anatech View Post
...... There is no way running your analogue sections off a supply with ripple and noise will ever be better than a nice quiet power supply.

-Chris
While that's generally true, there could be exceptions?
Like maybe the lower voltage of the regulated supply is having an impact?
Or the regulators are not working well?

It seems to be known that the I/V does not works as well on lower voltges. Why is that?
It sounds like time to get in there with a scope.
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Old 24th October 2017, 05:10 PM   #5
hollowman is offline hollowman  United States
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Default Time to experiment further ..

Quote:
Originally Posted by Chris888 View Post
While that's generally true, there could be exceptions?
Like maybe the lower voltage of the regulated supply is having an impact?
Or the regulators are not working well?

It seems to be known that the I/V does not works as well on lower voltges. Why is that?
It sounds like time to get in there with a scope.
Lots of possibilities, I realize! I'm betting on lower voltage.
May have to break out one of my small Amveco xformers to experiment.
BTW: I used a Tangentsoft TREAD regulator (it's based on LM317).
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Old 24th October 2017, 05:58 PM   #6
Chris888 is offline Chris888  United Kingdom
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There are many pitfalls in designing regulators.
Maybe a conventional IC regulator circuit or even a COTS psu would be a good intermediate step in evaluating the rest of the unit, for not too much expense or effort?
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Old 25th October 2017, 03:45 PM   #7
anatech is online now anatech  Canada
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No regulator for best sound?
Hi Chris,
Quote:
While that's generally true, there could be exceptions?
No, not with signal circuits. For a power amplifier, some see an improvement, but this wastes power. The best compromise is to regulate the voltage amp stage and allow the current amplifier stage to run off unregulated power. The voltage gain of the output stage is commonly just less than one, so noise will not be amplified. Amplifiers that are set up this way tend to have wider signal to noise ratio, and also seem to be open and airy sounding even when the bass is pounding away. The regulator insulated them from all / most power supply noise and variations. My Marantz 300DC is designed like this for example.
Quote:
Like maybe the lower voltage of the regulated supply is having an impact?
Or the regulators are not working well?
Could be either or both. With an oscilloscope, you could check the regulated voltage for signs of drop-outs.
Quote:
It seems to be known that the I/V does not works as well on lower voltges. Why is that?
Could be you don't have enough voltage compliance (range to maximum peak output voltage). It also could be the circuit that picks up the signal from the I-V converter.
Quote:
It sounds like time to get in there with a scope.
You're blind otherwise. I'd recommend a 20 MHz bandwidth as an absolute minimum. Buy the best you can afford, used is also a valid choice. I'm using a $20,000 oscilloscope (Agilent 54642D) that I could never afford. I saw one for about $1,000 and jumped on it. So whatever you end up with, as long as it's a good one, is a wise purchase and multiplies the dollars you spend. I had been looking for a few years.
"Chance favours the prepared mind"

-Chris
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Old 27th October 2017, 10:36 AM   #8
Chris888 is offline Chris888  United Kingdom
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Quote:
Originally Posted by anatech View Post
.....
Could be either or both. With an oscilloscope, you could check the regulated voltage for signs of drop-outs.
....
You're blind otherwise. I'd recommend a 20 MHz bandwidth as an absolute minimum. Buy the best you can afford, used is also a valid choice. I'm using a $20,000 oscilloscope (Agilent 54642D) that I could never afford. I saw one for about $1,000 and jumped on it. So whatever you end up with, as long as it's a good one, is a wise purchase and multiplies the dollars you spend. I had been looking for a few years.
"Chance favours the prepared mind"

-Chris
Definitely worth getting a scope if anybody is at all serious about analogue electronics.
I'd agree anything below 20MHz bandwidth is of limited use.
I would also say that a minimum of two channels will be useful quite often.
You can often get something that works on ebay for surprisingly little cash.
Just get something you can see working before buying it.
You can always trade up to a nicer instrument later.
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Old 27th October 2017, 11:11 AM   #9
Xoc1 is offline Xoc1  United Kingdom
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As the CD player is obviously built to a budget and has ripple on the voltage rails why not increase the capacitance after the rectifiers?
A simple substitution with some decent higher value low ESR caps should help.
I don't think I will bother with modding CD player outputs again, after building an external DAC which outperformed my CD player by a large margin, and allowed me to connect other digital sources such as the TV!
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Old 27th October 2017, 12:19 PM   #10
analog_sa is offline analog_sa  Europe
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Quote:
Originally Posted by hollowman View Post
I decided to regulate the stock unregulated +15v with an LM317 circuit (the LM317 was config'd to output 12v). While the ripple reduced several orders in magnitude, the sound quality worsened: bass became puny and dynamics became very weak.


Goodness. This could have been exciting news in 1985. Dunno what is your excise for learning it this late Perhaps frequenting the scope-heads forum a bit too much

These 3-pin monstrosities simply destroy the sound. Want an audible upgrade upon raw dc? Look at a nice shunt. Just mho of course.

Others think that properly built passive filtering is unmatchable with active solutions but at the cost of lots more iron and capacitance.
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