mic preamp modification

I recently bought a Marantz PMD670 -- a digital audio recorder. It seems
pretty good except the mic preamps have a pretty high noise level for a
recorder that is good otherwise. I'd like to see if I can make it
better. (It isn't broken, its a design issue.)

As a general description, the mic inputs are balanced XLR with
switchable phantom power. It only seems to work decently with pretty
high-level condenser mics. A lower dynamic seems to require cranking the
gain way up and the crappy noise floor becomes apparent.

One guy offers a mod for the preamps that improves things considerably.
link:
http://www.oade.com/digital_recorder...s/PMD-670.html
He only does it on stuff he sells and I already have mine. I asked him
if he would share anything about the nature of his change, but he
declined.

I don't have any schematic, but I opened the thing up and had a look at
what is in the preamp area. Assuming the signal flows sensibly across
the board, there are (per channel) two largish electrolytics. Then a
couple sot-23 devices (transistors I assume) marked 62Z. Then an HC4052A
analog mux. Then a JRC 2068 op amp.

I was surprized to see that 4052 so close to the input. Not sure what
exactly it is doing.

If I was just to replace the 2068 with something else, do you think it
would help? If so, what might be good choices for replacements?

Anyone know what the 62Z devices are? I did some searching but didn't
find anything that matched. Do you think that bears looking at?

What are the chances that the caps are contributing? I'm not sure what
these are. They have a symbol that is an S in a circle. They are marked
CE105 C above 0402. Is 402 the value? What does it mean?

Any thoughts about the HC4052A?

I know this isn't much to go on without a schematic. I don't have much
experience with quality audio circuits. I thought I would start by
asking opinions here about whether swagging out any of these parts is
worth the effort.

Thanks for any opinions.

Hello Rex,

FWIW, the 4052 is a dual 4:1 multiplexer. Sometimes also used to switch
resistors in and out for gain control. I don't know about this
particular opamp but check it's data sheet for noise performance. If it
is considerably worse than others a swap might help.

Anyways, you might want to print all the data sheets of the parts and
draw a schematic to understand how it works.

Phantom power sources can be a noise contributor but if so that could be
filtered.

Regards, Joerg

http://www.analogconsultants.com

rex wrote:

> I recently bought a Marantz PMD670 -- a digital audio recorder. It seems
> pretty good except the mic preamps have a pretty high noise level for a
> recorder that is good otherwise. I'd like to see if I can make it
> better. (It isn't broken, its a design issue.)

I'll guess it's a design issue. ;-)

> As a general description, the mic inputs are balanced XLR with
> switchable phantom power. It only seems to work decently with pretty
> high-level condenser mics. A lower dynamic seems to require cranking the
> gain way up and the crappy noise floor becomes apparent.
>
> One guy offers a mod for the preamps that improves things considerably.
> link:
> http://www.oade.com/digital_recorder...s/PMD-670.html
> He only does it on stuff he sells and I already have mine. I asked him
> if he would share anything about the nature of his change, but he
> declined.

Tuh ! Silly wanker.


> I don't have any schematic, but I opened the thing up and had a look at
> what is in the preamp area. Assuming the signal flows sensibly across
> the board, there are (per channel) two largish electrolytics. Then a
> couple sot-23 devices (transistors I assume) marked 62Z. Then an HC4052A
> analog mux. Then a JRC 2068 op amp.
>
> I was surprized to see that 4052 so close to the input. Not sure what
> exactly it is doing.
>
> If I was just to replace the 2068 with something else, do you think it
> would help?

NO. The 'front end' is what matters.

> If so, what might be good choices for replacements?
>
> Anyone know what the 62Z devices are?

That's a typical SMD code. There isn't enough space to fit the full part
number. Sorry !

> I did some searching but didn't
> find anything that matched. Do you think that bears looking at?

NO.

> What are the chances that the caps are contributing?

So close as next to nil that you might as well forget it.

> I'm not sure what
> these are. They have a symbol that is an S in a circle. They are marked
> CE105 C above 0402. Is 402 the value? What does it mean?

105 means 105 degrees C rating - that's fine. 0402 is normally an SMD
footprint size.

> Any thoughts about the HC4052A?

Nope.


> I know this isn't much to go on without a schematic. I don't have much
> experience with quality audio circuits.

I do !

> I thought I would start by
> asking opinions here about whether swagging out any of these parts is
> worth the effort.

Unlikely. Once the circuit topology is fixed there's usually bugger all that
you can do to improve it.

The Eastern guys don't have a clue how to make a good mic amp - apart from
Yamaha maybe ?


Graham

Joerg wrote:

> Hello Rex,
>
> FWIW, the 4052 is a dual 4:1 multiplexer. Sometimes also used to switch
> resistors in and out for gain control.

Erk ! Thinks *non linear resistance with applied voltage*.

> I don't know about this
> particular opamp but check it's data sheet for noise performance.

It's actually quite good.

> If it is considerably worse than others a swap might help.

It won't.


> Anyways, you might want to print all the data sheets of the parts and
> draw a schematic to understand how it works.
>
> Phantom power sources can be a noise contributor but if so that could be
> filtered.

Damn well shouldn't be a noise source unless the designer was mentally
defective !

Graham

Hello Graham,

>>FWIW, the 4052 is a dual 4:1 multiplexer. Sometimes also used to switch
>>resistors in and out for gain control.
>
> Erk ! Thinks *non linear resistance with applied voltage*.

Yes. The true audio freak would frown and at least use an SD5400 here.

>>Phantom power sources can be a noise contributor but if so that could be
>>filtered.
>
> Damn well shouldn't be a noise source unless the designer was mentally
> defective !

Then I guess I have diagnosed mental defects in two designers
life-to-date...

Both systems looked very expensive and "pro", BTW. Needed ferrites and
better electrolytics.

Regards, Joerg

http://www.analogconsultants.com

"Joerg" <(E-Mail Removed)> wrote in message
news:yMy5e.877$(E-Mail Removed).. .
Hi, Joerg.

> Hello Graham,
>
>>>FWIW, the 4052 is a dual 4:1 multiplexer. Sometimes also used to switch
>>>resistors in and out for gain control.
>>
>> Erk ! Thinks *non linear resistance with applied voltage*.
>
> Yes. The true audio freak would frown and at least use an SD5400 here.

In one project I worked on, one of the most competent and
conscientious engineers I've known used the 8:1 mux from
that family for a wide, RF (2 to 13 MHz), barrel switch
where noise and distortion were both critical parameters.
(The signal at that point was boosted enough that thermal
noise from the switch was not too harmful, and judicious
use of carefully biased followers eliminate most distortion.)

>>>Phantom power sources can be a noise contributor but if so that could be
>>>filtered.
>>
>> Damn well shouldn't be a noise source unless the designer was mentally
>> defective !
>
> Then I guess I have diagnosed mental defects in two designers life-to-date...

I expect you were more charitable than to do that.

> Both systems looked very expensive and "pro", BTW. Needed ferrites and better electrolytics.

Often, I think, appearance is the primary design
goal in expensive audio gear sold to consumers.

Best regards,
--
--Larry Brasfield
email: (E-Mail Removed)
Above views may belong only to me.

Hello Larry,

>>Yes. The true audio freak would frown and at least use an SD5400 here.
>
> In one project I worked on, one of the most competent and
> conscientious engineers I've known used the 8:1 mux from
> that family for a wide, RF (2 to 13 MHz), barrel switch
> where noise and distortion were both critical parameters.
> (The signal at that point was boosted enough that thermal
> noise from the switch was not too harmful, and judicious
> use of carefully biased followers eliminate most distortion.)

Being a cheapskate when designing stuff I often use PIN diodes with a
long enough carrier lifetime for RF muxing. The SD5400 is usually
reserved for more valuable tasks such as servoed gain or phase control.

>>>Damn well shouldn't be a noise source unless the designer was mentally
>>>defective !
>>
>>Then I guess I have diagnosed mental defects in two designers life-to-date...
>
> I expect you were more charitable than to do that.

Yes, one of my rules is not to place blame on a person. I just corrected
the problem and went on with life.

>>Both systems looked very expensive and "pro", BTW. Needed ferrites and better electrolytics.
>
> Often, I think, appearance is the primary design
> goal in expensive audio gear sold to consumers.

Oh yeah. I have seen gear in polished boxes with gold plated connectors
that cost hundreds yet contained less than 50 Cents in parts. The best
was a "compressor". It contained two 1N4148 diodes.

You can also see that effect in some lab gear. I remember the first time
I opened a "power splitter" that retailed in the four-digit range. It
contained a plain old hybrid with three resistors. Ok, they were laser
trimmed. But I have designed a lot of laser trimmed circuitry and it
doesn't cost an arm and a leg.

Regards, Joerg

http://www.analogconsultants.com

"Joerg" <(E-Mail Removed)> wrote in message
news:VLz5e.21285$(E-Mail Removed) m...
> Oh yeah. I have seen gear in polished boxes with gold plated connectors
> that cost hundreds yet contained less than 50 Cents in parts. The best was
> a "compressor". It contained two 1N4148 diodes.

This is too easy it's not even fair, but I love Monster Cables and how they
can even manage to take a regular old phone cord for dial-up Internet
connections and turn it into "Ultra-High Speed Internet Phone Cable"
(http://www.monstercable.com/computer...rnet%20Cables)...
Oh yeah, 3kHz and 30dB SNR clearly needs $2/ft cables!

> You can also see that effect in some lab gear. I remember the first time I
> opened a "power splitter" that retailed in the four-digit range. It
> contained a plain old hybrid with three resistors.

I've seen designs where cable TV power splitters were actually disassembled
and their parts then removed and soldered down to a different board because
the commercial alternatives (e.g., those from MiniCircuits) were actually
more expensive overall!

---Joel Kolstad

Hello Joel,

> I've seen designs where cable TV power splitters were actually disassembled
> and their parts then removed and soldered down to a different board because
> the commercial alternatives (e.g., those from MiniCircuits) were actually
> more expensive overall!

Done that, for lab experiments. It's just so tough to crack these
splitters open if you don't want the dreaded F-connectors.

Regards, Joerg

http://www.analogconsultants.com

On Fri, 08 Apr 2005 09:23:30 +0100, Pooh Bear
<(E-Mail Removed)> wrote:

>rex wrote:
>
>> I recently bought a Marantz PMD670 -- a digital audio recorder. It seems
>> pretty good except the mic preamps have a pretty high noise level for a
>> recorder that is good otherwise. I'd like to see if I can make it
>> better. (It isn't broken, its a design issue.)
>
>I'll guess it's a design issue. ;-)
>
>> As a general description, the mic inputs are balanced XLR with
>> switchable phantom power. It only seems to work decently with pretty
>> high-level condenser mics. A lower dynamic seems to require cranking the
>> gain way up and the crappy noise floor becomes apparent.
>>
>> One guy offers a mod for the preamps that improves things considerably.
>> link:
>> http://www.oade.com/digital_recorder...s/PMD-670.html
>> He only does it on stuff he sells and I already have mine. I asked him
>> if he would share anything about the nature of his change, but he
>> declined.
>
>Tuh ! Silly wanker.

Well, he won't work on mine since I didn't buy it from him, so I though
he might be willing to give me a general idea.

>
>
>> I don't have any schematic, but I opened the thing up and had a look at
>> what is in the preamp area. Assuming the signal flows sensibly across
>> the board, there are (per channel) two largish electrolytics. Then a
>> couple sot-23 devices (transistors I assume) marked 62Z. Then an HC4052A
>> analog mux. Then a JRC 2068 op amp.
>>
>> I was surprized to see that 4052 so close to the input. Not sure what
>> exactly it is doing.
>>
>> If I was just to replace the 2068 with something else, do you think it
>> would help?
>
>NO. The 'front end' is what matters.
>
>> If so, what might be good choices for replacements?
>>
>> Anyone know what the 62Z devices are?
>
>That's a typical SMD code. There isn't enough space to fit the full part
>number. Sorry !

Yes, I know. There are some lists online and I have been successful with
matching some in the past, but not this time.

>
>> I did some searching but didn't
>> find anything that matched. Do you think that bears looking at?
>
>NO.
>
>> What are the chances that the caps are contributing?
>
>So close as next to nil that you might as well forget it.
>
>> I'm not sure what
>> these are. They have a symbol that is an S in a circle. They are marked
>> CE105 C above 0402. Is 402 the value? What does it mean?
>
>105 means 105 degrees C rating - that's fine. 0402 is normally an SMD
>footprint size.

These are not smd parts. They are radial lead, about 5mm dia x 10 mm
long. It was hard to read them in the circuit but I didn't see anything
other than the 0402 that could be a value, so I assume it might mean
4000 somethings. 4 uF maybe? Didn't see any voltage rating though.

>
>> Any thoughts about the HC4052A?
>
>Nope.
>
>
>> I know this isn't much to go on without a schematic. I don't have much
>> experience with quality audio circuits.
>
>I do !
>
>> I thought I would start by
>> asking opinions here about whether swagging out any of these parts is
>> worth the effort.
>
>Unlikely. Once the circuit topology is fixed there's usually bugger all that
>you can do to improve it.

But the Oade guy I mentioned above seems to have found a way to get S/N
from 65 to 80 dB and distortion from 0.08 to 0.06. Getting the unit
apart is a pain, I did it to look at the board, so I think he can't be
doing too much to the circuit for what he is charging.

>
>The Eastern guys don't have a clue how to make a good mic amp - apart from
>Yamaha maybe ?

Is Marantz design done in Asia? I though they were based in Europe.