Increase Gain of Phono Preamp Possible?

Marantz P400,  P700 is almost identical,

3 stage direct coupled NPN with feedback city,

getting about 31 db stock, changed last transistor to 2N5088 (from 2SC1775a) and got it up to 34 db,

4 mv in, about 216 mv out,

vol knob has to go hi compared to CD , 

40 db would be nice,  don't want to screw up the RIAA,

thanks for any help!

circuit with DC volts>

Wouldn’t it be easier to pad the CD input?

CJ said:

Marantz P400,  P700 is almost identical,

3 stage direct coupled NPN with feedback city,

getting about 31 db stock, changed last transistor to 2N5088 (from 2SC1775a) and got it up to 34 db,

Click to expand...

That is not the correct strategy to change gain (transfer function) in a NF circuit.

If the transistor swap makes a difference, perhaps the old transistor was bad or the design was marginal.

4 mv in, about 216 mv out,

Click to expand...

at what frequency Kenneth?

RIAA EQ has a roughly 40dB gain change over the audio bandwidth (20dB more gain at 20Hz, 20dB less at 20kHz).

vol knob has to go hi compared to CD , 

Click to expand...

how sad  :

40 db would be nice,  don't want to screw up the RIAA,

thanks for any help!

circuit with DC volts>

Click to expand...

You can change feedback values, but lots of moving parts that will interact...  Making 620 ohm smaller will increase wide band gain,,, BUT in series with 47uF cap so also changes LF pole. As you mentioned the open loop gain may already be marginal.. if changing one device changes gain. :

Relax, drink a cool one... no drink a cold one.

JR

Higher output stylus??

Actually just found this....

Rebuilding a Marantz P400 Phono Board - BG045

https://www.youtube.com/watch?v=LHkV7o2ExKw

maybe some info in it...?? or not....

> vol knob has to go hi compared to CD

This is normal. All CDs are "cut LOUD". Phono cutters have to use judgement; more loud is less time per side, and you complained if you didn't get enough music per side. But a few disks have quite enormous peaks, so you can't just whomp-up the playback gain or peaks go "f-t-t-f".

The 1KHz gain is essentially the ratio of the two resistors shown. Getting gain by measurement is tricky, and I'd trust the resistors over your measurements. As said, make the 620 smaller. Try 470, maybe 390, but I'd expect 330 to cause clipping on *some* of your records. The pole with 47uFd is still subsonic, benign, even good.

PRR said:

> vol knob has to go hi compared to CD

This is normal. All CDs are "cut LOUD".

Click to expand...

Which is why it will probably sound better to pad the CD input so it doesn’t cream the input stage with level.

Or get a DJ cart with a high output level like a Shure 44.

Or get a Denon 103 with a 1:10 step up transformer. It magically needs a 47k termination. I use one and love the sound.

> pad the CD input so it doesn’t cream the input stage with level.

The AUX (no CD!) inputs go to the Bal/Vol pots. Nothing to overload.

Taking a "normal needle" as 5mV nominal level, gain of 76 makes 380mV. The CD standard aimed for the 316mV of nominal Hi-Fi interfacing. But vinyl can be well below or above "nominal". So can CD, but Loudness Wars (and look-ahead digital processing) mean that essentially all CDs are cut far on the hot side.

While 5mV is nominal for 33rpm, peaks to 70mV exist on some records. With stock gain of 76, this is 5V, and at 33V this preamp will be unclean at 10V output. Reducing 620r to 330r will make some few passages output 10V, clipping in the preamp.

I could agree with a 6dB mod on the AUX inputs when used with CD, essentially countering Loudness Wars effects. Since all CD decks are low-low impedance, this could be two 3k resistors on each AUX jack.

There is a converse problem when some cellphone headphone jacks are fed to a Hi-Fi system. 300mV in 32 ohm phones is "dangerously loud", and pose liability questions. I have not heard of a deafened kid suing his cellphone (they usually just find louder headphones), but many cellphones won't do 300mV peak well. The high-level input sensitivity of this model is rated 180mV, so you can probably get most of the 70 Watts claimed with a cellphone.

wow, thanks for all the replies! Great info!

I will keep this thing stock maybe try a resistor change (620) for kicks,

this thing clips pretty ugly with a  notch at the top of the wave form, changing the 100 K pot merely changes the clip point,

i watched that youtube guy and stuck in the caps, diodes and transistors,

there seems to be a prejudice against the flat transistors in the first two stages, (2SC1344)
we replaced those with the KSC845's  ,  claims were noise and leakage from the old ones but they tested great for matched hfe and V-eb volts an V-c.


some folks use an external preamp with selectable  gain to match up the records with the receiver