Stereo Phono Cartridge to both Stereo and Mono Amps

Hi,

There are lots of ways to approach this, ranging from simplistic to complex : )

I don't know phono very well, but as I understand it may need a preamp first, with phone input, to sort out the cartridge response?

If so, then from the output of the stereo preamp, you could try adding a couple of resistors (perhaps 1 kohm might work) as shown in the sketch below, and just see what happens. It might be fine, or it might not, depending on the output and input capabilities of the preamp and the power amps respectively. But it's a (very low-cost) start, and if you're not happy with this then you could try something else. 

A more advanced option would be to implement a proper mixer with controls. Yet another option could be to do it in DSP if you need the mono output to have a different response (e.g. low-pass for a bass response on that channel). That's not particularly difficult, but of course is a bit more of a major project compared to the two resistors.

Shabaz explains the principle in his message. (Although I think 1k resistors may be too low for some pre-amps)

The problem is that a good mono mix is not made by adding the two channels of a stereo mix. The two resistors do as good a job as anything else but it may well not sound quite right.

If you mean to feed two different amps you will need the preamp with RIAA equalization (if its a magnetic cartridge) or a high input impedance preamp (if its a ceramic cartridge).

Can you describe your simple methods and the cartridge ?

Are you willing to make the pre-amp yourself  ?

MK

The turn table is of a 1953, Rock-Ola 1446, AKA Fireball Comet 120.  The cartridge is a TN4 and the tone arm wire is a three conductor 30AWG Shielded wire. The internal mono amp assembly is a Rock-Ola 18640-A Tube Amp with an internal 12" speaker and horn. The external Stereo amp is a Sony STRDH190 2-ch Home Stereo Receiver with Stereo Phono Input that I am using. 

One of the simple methods I referred to was similar to what Shabaz described (1K Ohm) accept no preamp is used where shown. The mono sounded fine but stereo signal lost all its depth and dimension.

The second method used I replaced the two resistors with two Iso Transformers (EI14/EI-14 Audio Coupling Isolation Transformer Audio Transformer 600:600 1:1) . When connected he Stereo signal strength dropped so low I had to turn the volume to 10 on the Sony and it was very tinny sounding. Due to the poor signal strength I was unable to determine any loss in stereo dimension. This might require a preamp before the Iso transformers. 

I'm pretty sure that the TN4 is a ceramic cartridge - it needs a load of at least 1M (and some say > 5M is much better). Without a detailed spec (which I couldn't find) it's not possible to be specific.

Your isolating tramsformers are totally unsuitable (as you have found).

The Sony amplifier is not intended for use with a ceramic cartridge and will not work well although, to some, it might be tolerable.

You need to buy or make a suitable amplifier. (I couldn't find one to buy with a quick search.)

Here's a link for a simple design:

https://www.eeweb.com/preamp-stage-for-ceramic-phono-cartridge-or-violin-pickups/

Beware, there are some very sill designs on the web as well !

If you want to try building one then I can help a bit more.

MK

MK,

I am interested in building one. Further Understanding: Is this preamp part of the basis needed to separate a usable mono and stereo signal then?

Tom

The signal from the ceramic cartridge needs to work into a very high impedance,  most amplifier inputs are not suitable unless actually intended for this.

To make a mono signal you could just join the two outputs of the cartridge. These could connect directly to the valve (tube) amplifier but I'm not sure where the volume control is - the bass might be very weak due to the input impedance of the amplifier, but might be OK if this was what the amp was built for. This solution doesn't help with your Sony amplifier.

A better scheme is to build a two channel pre-amp and have stereo outputs for the Sony amplifier (use its AUX input not the phono) and a mono output for the valve amp.

Before I make a suggestion of a suitable circuit it would be good if you could give me an idea of your skill level:

1) can do pcbs and surface mount

2) OK with perf board and through hole soldering

3) Not happy about soldering

MK

Thanks MK, 

The quick answer is 2, I'm OK with perf board and through hole soldering (I have done surface mount repairs on game systems using a microscope for this work but I have not created any PCB's since school in the 80's). Through hole will be great.

Tom

Below: This is a diagram of the Tube Amp. I have replaced the caps, resistors and poorly performing tubes. Sounds great!

The amplifier has a reasonable input impedance (1.5M). There doesn't seem to be a volume control anywhere, otherwise the ceramic cartridge could drive it directly (although there might not be enough gain). Is there a volume control off to the left somewhere ?

MK

MK,

Yes, here it is.

I should also mention there is a mute circuit for when the mechanicals are picking up and returning the record disk. In the tube amp circuit this is accomplished by grounding the B terminal. I am hopeful there is a way this can be incorporated into the stereo amplifier solution.

Thank you kindly for your time and consideration. I am much appreciative, thank you. - Tom

Hi Michael,

I found a reference to ceramic pickups on http://www.douglas-self.com/ampins/library/ampartew.htm which directed to a Wireless World article.

In the August 1971 PDF issue of Wireless World page the author of the article has a 'economy version' circuit (reproducing here since the PDF version is freely available):

and also a "luxury version" which is claimed to be compatible with many ceramic cartridges:

I know nothing about any pickup tech ceramic or otherwise! : ) so I have no idea if these circuits, or front end of these circuits, are any good or not.

Nice that they use only easy-to-find BC109 though! The author must have had a whole bag of them to use : )