Turntable is too loud - Rotel RA-314 with Technics SL-D2

Hello,


This is my first post here in the diyaudio forum and I hope I am posting this thread under the right section. (Note to admin: Feel free the move the thread to the proper section of the forum, if necessary)

First about the setup:
Amplifier/Speakers: Rotel RA-314 + Hitachi HS-540 8Ohms

Turntable: Technics SL-D2 with Audio Technica MM AT-VM95E cartridge.

The problem:
Turntable is too loud (or MM input Z of the amp is too low), so that when I am playing vinyl, I have to turn the volume knob all the way down to the <1 over 10(max) so that the house does not collapse on me (ok, little bit exaggeration here).


Normally I use the amp with a media PC connected to the TV via HDMI and the 3.5mm headphone output of the TV is connected to the AUX of Rotel. Speaker out volume of the TV is at max and the PC volume (Windows 10) around 10/100 and Rotel volume is around 2-3/10.

So my aim is keeping the volume knob of the Rotel where it is and only be able to turn the source knob from aux to phono. So that I will be able to listening from the turntable while the volume (gain) of the amp is not ridiculously low and enjoying the amp a little bit more.

Until now I tried a pair of RCA inline attenuators, but they only reduced the sound quality.(In comparison to no attenuator and the volume of the amp below 1/10). I also built a box with a Pot inside and it only introduce the noise and hum. (Development goes further on that)

Now I am considering whether I can modify the input stage (preamp) in the Rotel so that I can reach what I want or not. But my electronics knowledge has been reached to its limit really fast, as I started looking into the input stage topology of the Rotel.

Disclaimer: I am neither a amp builder or an E/E Engineer. I have a basic understanding of electronics and op-amp theory (so long an opamp have 5pins), also very good soldering skills.

There is a screenshot of the Rotels preamp stage at the attachments from the service manual. It appears to me too complicated, due to the lack of experience. So is there an option for modification here? It seems like, as if I increase the value of the R701 R or R401 (for right channel), it might do the job, but I am probably wrong.

Another screenshot from the service manual is the gain diagram of the amp (also at the attachment). There I see the input is 2.2mV whereas my phono cartridge puts out 4mV. Am I also wrong here?

Back to input stage of the Rotel. I have seen some basic phono preamp designs and videos and in one of these someone told that one has to put a resistor between the ground and non-inverting input of the op amp as a driving (or load) resistor for the phono cartridge (4.7k for the my cartrige). I only see R701 as 3.3M.

All in all I am little bit confused right now and appreciate any advise or help.

Thanks and Regards,
Oz
 

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Welcome to diyAudio :)

R701 is simply a high series feed for using a ceramic cartridge. It plays no part here.

The gain of the stage is set by R411/412 and increasing that resistor decreases overall gain but while that hold true this is not a standard opamp as we know them and it is really a simple single rail preamp in an IC package.

So... you could experiment with that resistor or you could try splitting R421 and making a divider (say two 15k's it is not critical) and taking the output (to R423) from the divider.

Changing the gain resistor may get what you want but I would want to look at the DC conditions and make sure they were still OK and also that the RIAA response was still good.

R403 (100k) in parallel with R409 (100k via C401 at AC) essentially set the input impedance at about 50k.
 
Thanks Mooly and Bonsai for your answers,


So... you could experiment with that resistor or you could try splitting R421 and making a divider (say two 15k's it is not critical) and taking the output (to R423) from the divider.


Do you mean removing the R421 and soldering 2x15k resistors parallel to each other where R421 used to be? Unfortunately I could not understand what you meant there.



Strange though that the gain is so high. I believe a lot of these earlier Japanese amps were spec’ data 2.5mV in on MM. that still would not explain the very high output though. Is the amp ok (ie correct volume) on the other inputs?


I have written in my first post that I use aux with TV Headphone Output, while the media PC is set to 10% Volume (Windows 10). Therefore I can operate the amp at its volume settings 2-3 out of 10 (volume knob) since the last 5 years. If you suggest a measurement method which we can conclude that the other inputs are ok or not, I can happily try and report it.


Regards,
Oz
 
Hi Ralph,


I'm wondering whether someone in the past has added a 'preamp' inside the turntable?
Have you tried plugging the phono plugs from the TT into another input on the Rotel? perhaps an 'aux' input?


when I plug the turntable into the AUX of the amp, I have to crank the volume knob of the amp to over 7 or 8 to be able to achieve the same volume. Therefore I believe there is no preamp bodged into the turntable.


Regards,
Oz
 
Administrator
Joined 2007
Paid Member
Do you mean removing the R421 and soldering 2x15k resistors parallel to each other where R421 used to be? Unfortunately I could not understand what you meant there.

Remove R421 and replace it with two series connected resistors.

Transfer R423 to the junction of the two resistors. The ratio of the two resistors determines the attenuation. If both are equal then you get 50% of the voltage at the junction.

If you make the top resistor lower in value compared to the bottom one (for example 10k and 22k) you will get more signal, reverse the positions and you get a lower signal.

The values are not very critical although if you make the combination to high then high frequencies may be attenuated slightly and if you make the overall value to low then the circuit might have trouble driving the lower resistance values.

Voltage divider - Wikipedia
 

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