Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Wed 16, 2019 6:48 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 7 posts ] 
Author Message
 Post subject: R-390 audio connection question
PostPosted: Sep Mon 24, 2018 2:46 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20267
Location: Warner Robins, GA
I have an R-390 and I am thinking of connecting the line audio out to a stereo for recording purposes if I so desire to record something I've received.

I have a Hammond 600 ohm to 4/8 ohm transformer connected there for a pair of 32 ohm stereo headphones with the phones jack on the receiver replaced with a stereo one wired for mono.

The stereo is on the opposite wall from the receiver which is about 12' away and would require a little over 25' of cable to reach.

The receiver is on a separate ground which goes to a ground rod.

I have an Edcor XSM 10K/600 transformer.

Is it possible to connect the receiver to the stereo using the phones output which is unbalanced or would it be best to run a balanced line to the stereo and use the XSM 10K/600 with the 600 ohm side as the primary to do the balanced to unbalanced conversion?

Also should I install a 11K resistor across the 10K winding to keep the impedance the receiver sees at 600 ohms since the McIntosh C24 preamp I am using has a 250K input impedance on its aux inputs which would be 125K with the right and left channel input parallelled?

The idea is I want no buzz, hum ETC... from that input nor do I want the receiver's ground to be connected to the stereo's ground.


Top
 Profile  
 
 Post subject: Re: R-390 audio connection question
PostPosted: Sep Mon 24, 2018 8:47 pm 
Member
User avatar

Joined: Nov Sat 07, 2009 11:37 pm
Posts: 2184
Location: Sayreville, NJ 08872
Personally, I would take the R-390 audio output off the detector with a coupling capacitor and roughly 10K resistor and feed it into the AUX input of one channel and set the front panel switch to MONO so that it comes out of both speakers. Lacking the MONO position, use a Y-connector on the AUX left/right inputs. Your way requires diddling with the R-390 volume control so as not to drive the signal into the stereo into distortion plus you have to deal with any audio compensation parts in the R390 audio circuity.

_________________
Pete, WA2CWA - "A cluttered desk is a sign of genius"
http://www.classicradiomanuals.com


Top
 Profile  
 
 Post subject: Re: R-390 audio connection question
PostPosted: Sep Tue 25, 2018 1:24 am 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 273
Location: San Jose, Ca.
Good advice. The R-390 has 2 audio transformers in the path of both outputs, further narrowing the audio response output to voice. If you'd like the best fidelity possible (and it would be much much better), connect the audio out to the diode load. And, since you are going to use a long coax to get the signal to the Mac preamp, you should drive the coax with a low impedance so as to minimize treble roll off due to the capacitance in the coax. The output impedance at the diode load is about 60k, so I recommend a 100k to 600 ohm transformer. This transformer should have a good response (20 h - 20 kh +/- 1 db) if you want good fidelity. I know that the power will never be very high in the transformer, but in order to minimize hysteresis distortion from being overdriven, it should be capable of at least .5 watt. Because the diode load has a DC component to it, connect it to the transformer with a .1 mfd cap @ 50 vdc. Have fun.

Regards, Larry


Top
 Profile  
 
 Post subject: Re: R-390 audio connection question
PostPosted: Sep Tue 25, 2018 1:30 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20267
Location: Warner Robins, GA
I thought about that, but then I would not have the audio filters of the R-390 available.

The preamp shorts the input jacks of the input that is not currently selected so that would take the 10K resistor to ground if that source was not selected.

I went ahead and tried the transformer and it works great.

I put a 10K resistor across the secondary so the impedance would be 600 ohms if the transformer is disconnected from the preamp.

Because of the input shorting I installed a 100K resistor between the transformer and the preamp. I used two RCA cables to make the connection so that I didn't have to operate the stereo/mono switch.

Works great so far and has no hum.

I would rather not do the diode load suggestion as the receiver's ground would then be tied to the stereo's ground which would be tied to the house ground through the USB sound card that is connected to the desktop I have.

Plus that would mean about 25' of unbalanced cable which would be susceptible to hum pickup given the cable is run along the floor and is near power cords.


Top
 Profile  
 
 Post subject: Re: R-390 audio connection question
PostPosted: Sep Tue 25, 2018 1:40 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20267
Location: Warner Robins, GA
Lar H wrote:
Good advice. The R-390 has 2 audio transformers in the path of both outputs, further narrowing the audio response output to voice. If you'd like the best fidelity possible (and it would be much much better), connect the audio out to the diode load.

Regards, Larry


Already did away with the audio filter driver transformer by going with a cathode follower using a 25L6 and the 12AU7 section that was once the driver for the transformer.

The audio from the line audio out is acceptable and aside for a slight bass loss due to the line audio out transformer it is ok as if I record anything it might be a shortwave station or something else where ultimate bass response is not necessary.

Sometimes I find the bandwidth control isn't enough to give good sound so I usually leave the audio response filter on MED which makes a lot of stuff more listenable.

Lar H wrote:
And, since you are going to use a long coax to get the signal to the Mac preamp, you should drive the coax with a low impedance so as to minimize treble roll off due to the capacitance in the coax. The output impedance at the diode load is about 60k, so I recommend a 100k to 600 ohm transformer. This transformer should have a good response (20 h - 20 kh +/- 1 db) if you want good fidelity. I know that the power will never be very high in the transformer, but in order to minimize hysteresis distortion from being overdriven, it should be capable of at least .5 watt. Because the diode load has a DC component to it, connect it to the transformer with a .1 mfd cap @ 50 vdc. Have fun.

Regards, Larry


Already had the 10K to 600 ohm transformer on hand and would have to buy that 100K to 600 ohm transformer.

Here's a short video of the audio.

https://www.youtube.com/watch?v=GZca74bSOIM

EDIT:

Noticed something.

I usually leave the preamp in tape monitor on and I have a small buffer inside so the record out can feed various inputs selected by a DBX 200 program route selector.

The audio from the R-390 is at one level with the tape monitor on and at a lower level with the tape monitor off. Figure that with the tape monitor off the impedance is lower so more audio is dropped across the 100K resistor.


Top
 Profile  
 
 Post subject: Re: R-390 audio connection question
PostPosted: Sep Tue 25, 2018 4:27 am 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 273
Location: San Jose, Ca.
I think you're right about the change in impedance with the mon switch on or off causing the difference in audio level. You could reduce the 100k resistor and add a 20k to ground to reduce the input impedance and keep the level a little more equal.

Glad you got it working.

Using the transformer connected to the diode load does not require the 2 devices be grounded together. It provides the isolation needed.

Regards, Larry


Top
 Profile  
 
 Post subject: Re: R-390 audio connection question
PostPosted: Sep Tue 25, 2018 6:00 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20267
Location: Warner Robins, GA
The transformer yes.

I could reduce the 100K resistor but that will cause the impedance to drop a good bit below 10K which will make the 600 ohm line be of lower impedance which might affect the output when using headphones. Probably not by much though.

Doing the diode load will make the audio filters not usable.

Now there is a resistor in parallel with the local and line level controls which is 1.2K and with the two 2.5K pots it equals 612.24 ohms impedance. If I can find a way to get a terminal on the rear to connect to that point I could get a transformer with a 1.2K primary (in place of the resistor) and a 600 ohm secondary then at the preamp I could install a 600 ohm resistor across the line and use a higher value series resistor to match the audio level to what the preamp requires while keeping the impedance from dropping any when the input is not selected.

Best I can do is a 2.4K to 600 ohm transformer which would require a 2.4K resistor across its primary so that the impedance the audio filters see is still 612.24 ohms.

That way I still have the audio filters available when needed plus I then have better audio quality.

Now because of my work with sound systems and knowing how balanced lines reject interference I'd be tempted to keep the line balanced and just use the transformer I currently have to take the 600 ohms impedance back to 10K at the preamp. Will also keep the impedances right to ensure the best sound quality.

Only way I know to get an audio line out for the transformer is to tie the common of the two secondaries of the line output transformer together in the audio chassis then remove the link on the rear and use one of those terminals for the transformer primary to connect with one side going to ground.


Top
 Profile  
 
Post New Topic Post Reply  [ 7 posts ]  Moderator: Sandy

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 9 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB