SP600 HI FI AUDIO!

[ke7trp]

Today while waiting for my Dog to get out of surgery.  I Decided to try to run HI FI audio out of the SP600. I was told by three hams that you cannot do this like you can with the R390A. 

Bill, w7HD sent me an email and said it would work fine.

I used a 470K ohm Resistor then into a 10UF NON polorized cap.. I tapped the resistor off the DIODE on the back of the RXer.  All I had on hand was a vintage McIntosh MA6100 preamp/amp.  I ran the audio into the AUX jack of the MAC, Then into a Paradigim reference monitor speaker. 

OH MY GOSH!!!!!!  I repeat OH MY GOSH!!!  The audio is just flat out incredible.  I cannot distigiush this from FM!  I have bass, treble and loudness.

This to note:

The volume control on the reciever does not effect the audio level of the outboard audio amp and speaker.

If you leave the normal speaker hooked up to the RXer, You can use that speaker also with the volume control on the Reciever as normal!  If you want HI FI, Just turn on the Stereo amp and you got it.

I really wish I did this a year ago.  Cant wait to hear the guys on this setup!

Instructions here:    http://www.r390a.com/html/diode_load.html

Clark

[K1JJ]

Hi Clark,

Yes, it is incredible to hear, huh?

I also used the SP-600 off the detector for years into a big hi-fi amp with four speakers mounted on the four corners of the ceiling/walls.  Big sound.

I found I needed to add an FET buffer stage between the detector output and the amp.  Otherwise, when swept with audio, there was some loading down of the detector causing a slight flat topping look to a sine wave.  I used a 3 meg cathode follower circuit and that did the trick. Check yours to be sure 470K is enuff buffering.


You mentioned you're suprised how good it sounds... as I wrote in the "Five Hurdles" article below, many AM stations I've visited had good receiving audio, but were missing the truly hi-fi sound of deep lows and extreme highs. The only way to get this is off a detector, into a big hi-fi amp and big speakers that are all designed for the typical FM stereo system.  It does make a difference.  However, when cornditions get rough, you will be better off listening in headphones with narrower bandwidths.

T

The Five Hurdles:
http://amfone.net/ECSound/K1JJ.htm

[ke7trp]

This worked for me. I have an IF scope and put it on an AF scope.  Looks clean to me Tom.  I have the scope integreated into the station anyways.  I will read your article in a bit!

C

[W2XR]

Today while waiting for my Dog to get out of surgery.  I Decided to try to run HI FI audio out of the SP600. I was told by three hams that you cannot do this like you can with the R390A.  

Bill, w7HD sent me an email and said it would work fine.

I used a 470K ohm Resistor then into a 10UF NON polorized cap.. I tapped the resistor off the DIODE on the back of the RXer.  All I had on hand was a vintage McIntosh MA6100 preamp/amp.  I ran the audio into the AUX jack of the MAC, Then into a Paradigim reference monitor speaker.  

OH MY GOSH!!!!!!  I repeat OH MY GOSH!!!  The audio is just flat out incredible.  I cannot distigiush this from FM!  I have bass, treble and loudness.

This to note:

The volume control on the reciever does not effect the audio level of the outboard audio amp and speaker.

If you leave the normal speaker hooked up to the RXer, You can use that speaker also with the volume control on the Reciever as normal!  If you want HI FI, Just turn on the Stereo amp and you got it.

I really wish I did this a year ago.  Cant wait to hear the guys on this setup!

Instructions here:    http://www.r390a.com/html/diode_load.html

Clark

Hi Clark,

I have been doing this for probably 25 years with my SP-600, and later with my R-390 and R-390A.

I do not see the need for the 10 uf DC blocking capacitor, as there should not be any DC at the detector output. I think Chuck Rippel of R-390A rebuilding fame was the one who started this need to include the large value cap, and like I said, I believe it is superfluous. The way I look at it, in good audio circuit design, any capacitor you can eliminate is always a good thing, and electrolytic caps are invariably the worst sounding caps for audio coupling and bypassing, etc.

Before you attempt to eliminate that cap, check first to ensure that there is indeed no DC at that point in the circuit, just to be safe.

In my SP-600, the 6V6 audio power amplifier stage was removed and a 6SN7 was installed in it's place. The detector output is connected directly to the grids of the 6SN7. The 6SN7 functions as a high input impedance voltage amplifier that drives a UTC A-25 plate-to-line xmfr, so my modified SP-600 provides a balanced 600 ohm line-level audio output. In the case of my R-390 and R-390A, the diode load output is connected directly to the 100K input impedance of an Ampex 351 vacuum tube record/playback amplifier that I use as a audio preamp for my receivers, and it also provides a balanced 600 ohm line-level  output.  Both of these set-ups work FB and provide true hi-fi sound, and they drive an Altec-Lansing tube power amplifier with the output stage triode-connected, and that in turn drives an Altec-Lansing 15-inch coaxial duplex speaker in a large bass reflex enclosure.

To my ears, at least, it does'nt get too much better than this.

I could never listen to hi-fi AM with the internal audio of even the best-grade general coverage or communications receivers; so much fidelity is lost due to poor quality output xfmrs, pentode output stages (single-ended or push-pull), lack of negative feedback to help linearize the output stage, etc.

Your mileage my vary, and strictly my own opinion.

73,

Bruce

[ke7trp]

I will play around with it. Thanks for the info.  We ended up putting a small Marantz 1060 inline. Its much smaller and lighter. Sound is still good. Not McIntosh good but that amp has to be sold.

Clark

[W7SOE]

I am going to do this to the r-390a tonight!  I think I will feed it to the phono input of my Hali SX-42 and r-42 reproducer, just to keep it tubular.

Recently I hooked up my scope to the IF output of the R-390A, this was also a great revelation!

73

Rich

[W2XR]

I am going to do this to the r-390a tonight!  I think I will feed it to the phono input of my Hali SX-42 and r-42 reproducer, just to keep it tubular.

Recently I hooked up my scope to the IF output of the R-390A, this was also a great revelation!

73

Rich

You may possibly have a little too much gain in the phono input section of that receiver, but try it and you'll find out soon enough. The detector output voltage from these receivers at full modulation is typically on the order of 1 to 2 volts across a 100K ohm or higher load.

73,

Bruce

[K5UJ]

Clark it's sweet isn't it.   I find (on the low bands at least) it's best in the daytime when there isn't a lot of activity nearby so I can work a station with no QSOs within 10 KHz.  Then I can crank up the amp and let the product detector wide-as-a-barn-door audio fly.  At night, storm static and QRM make it much less enjoyable so for me, "communications grade" passband and cans are usually the norm.   My favorite operating time is daytime--big audio and buzzarding on the dipole.

Rob

[N2DTS]

Been doing that for years, partly for the fidelity, and partly to have ONE volume control and speaker to worry about, even if I had a mess o receivers, plus it gives you a nice line level recording output and tone controls.

95% of receivers have really poor audio stages.

Brett

[W1AEX]

Yah, the SP600 definitely will produce the big jukebox sound in the wide bandwidth position. I used the technique suggested to me by Steve QIX back in the 80's and picked off the audio at the wiper of the SP600 volume control pot and fed that out through the back to an amp with a nice pair of push-pull 6550's driving an old KLH acoustic suspension floor speaker with a 10 inch woofer. That way you can control the audio level with the SP600 volume control. It really sounds beautiful on the BCB when you find a station pushing nice audio. It could really reproduce the famous Radio Moscow hum nicely too!

[ke7trp]

I now have the best of both worlds here.  I left the Halli R46B speaker hung on the wall. Its hooked to the SP600 as normal. This is controled through the normal Volume knob.

Then, If I want HiFI, I can switch on the little marantz and turn its volume up through the monitor speaker. 

What I find interesting now that I have good sound and an IF scope is how Shitty some AM Broadcast stations sound. Man are they Dry and narrow.  Only a few are big and wide now.  I have not used it much on HAM bands as I was busy caring for this dog of mine. 

Maybe tomorrow I can catch the guys on 7293 and have a listen!

Clark

[K5UJ]

yeah some of the bc stations have let things go because there is little incentive to sound good since 20 KHz pass band rx are probably one in a million out there on medium wave AM.   Many if not most are around 6 KHz passband.   If you can find a station that still programs music you probably will be getting better audio.   

Is the Marantz a tube amp?  I find the best result is with some kind of tube AF amp and a fairly big speaker, at least a "bookshelf" type hi-fi speaker that needs around 20 w. drive or more.   One advantage of a tube amp is that you avoid the problem of tx rf getting into it and rectifying and coming out via the speakers as noise garbage when you transmit.   

Rob

[ke7trp]

Most BC stations are less then 8KC wide now.  They are solid state boxes.  One I know is a Harris 1kw unit in town. It sounds like hell. 

I found one that was nice and wide.. Very FULL audio.. But it was Gospel.   

The Marantz is a soldi state amp. Its rather compact but still hifi.  It seems to be working great. I have tube amps here.  Dynacos and some real high end stuff. I just did not want anymore gear in this small room!

C

[iw5ci]

i cannot find a non polarized cap can i substitute it with other type of condensers?

[Ed/KB1HYS]

If there's no DC component on the output, any electrolytic should work.  I used a 10uF polarized electrolytic condenser with a 470K resistor for my R-390A and it works FB OM!  Even just feeding the output to a pair of cheap computer speakers with built in amp it sounded better than the audio out.

If there is a DC component, then you can still use a polarized cap just make sure the polarity matches the DC on the output.

[W2XR]

If there's no DC component on the output, any electrolytic should work.  I used a 10uF polarized electrolytic condenser with a 470K resistor for my R-390A and it works FB OM!  Even just feeding the output to a pair of cheap computer speakers with built in amp it sounded better than the audio out.

If there is a DC component, then you can still use a polarized cap just make sure the polarity matches the DC on the output.

Hi Ed,

Just out of curiousity, if there is no DC present at the output, why would any kind of blocking capacitor be required in this application?

I know Chuck Rippel advocated the use of the 10 uf non-polar cap with a 470K loading resistor, but I could never understand his rationale for that.

I have never used a blocking cap at the diode load output of my SP-600, R-390, or R-390A. There is no DC component (or should not be) at the diode load output of any of these receivers. I just feed the audio into a preamp and she works great. In audio, it's a good thing any time you can eliminate a capacitor, particularly the evil-sounding electrolytics.

I think the only potential issue is the load impedance of the audio amplifier the diode load is working into. Probably anything >100K ohms, which is typical for the line-level input in many tube and solid-state preamps would be fine. If the load Z is too low, then you will have frequency response anomalies, and a loss of output. A simple buffer circuit with unity gain would take care of that, such as a cathode follower, FET amplifier, etc.

Just my $.02.

Thanks & 73,

Bruce

[Ed/KB1HYS]

Bruce,

Yes, you're right about the cap, though I don't think it hurts to have one in there.

I had thought that the resistor would be required though to act as a load for the diode just in case the output gets shorted to ground - like when jacking into the amp or??

[w3jn]

There certainly *is* DC voltage on the diode load terminals on the R-390/390A.  In fact, that's the test point specified in the manual to attach a DC meter during alignment.  The manual specifies -7 V on the diode load at 4 microvolts input as I recall.  Examining the schematic shows that the diode load terminals are connected thru an RF choke to the secondary of the last IF xformer.  THe other end of that winding goes to the detector "diode" (actually 1/2 a 12AU7) whose cathode is grounded.  Obviously the DC voltage here will be proportional to the signal strength.  The diode load voltage can go quite high on strong signals; of course it's limited by the AVC action.  If the AVC is off it can swing to -60V or perhaps more.

It's always a good idea to isolate the radio from an external amp with a cap.  You never know what kind of input circuit an amp has, and you certainly don't want to present a DC load on the diode load output, as it's a high impedance circuit.

This would be true of just about any radio.  That's why there's a coupling cap between the detector and the first audio; you don't need the rectified carrier voltage getting into the grid of the first audio as it will dramatically and dynamically change the operating characteristics of the tube.

In any event you don't need a non-polarized cap to couple it to your audio amp.  A 10 uF electrolytic with the "-" end going to the diode load will work fine.  If you're feeding into a high impedance amp you can reduce the value to 1 or even .1 uF.  The reason for the resistor on the other end of the cap is to provide a discharge path for the cap.  If the amp has a capacitor in series with the input the cap won't have anywhere to dischage itself, and it's gonna cause a huge "thump" if you connect/disconnect it.

[Steve - WB3HUZ]

It is the output of a diode rectifier.  Why wouldn't there be DC?

[W2XR]

There certainly *is* DC voltage on the diode load terminals on the R-390/390A.  In fact, that's the test point specified in the manual to attach a DC meter during alignment.  The manual specifies -7 V on the diode load at 4 microvolts input as I recall.  Examining the schematic shows that the diode load terminals are connected thru an RF choke to the secondary of the last IF xformer.  THe other end of that winding goes to the detector "diode" (actually 1/2 a 12AU7) whose cathode is grounded.  Obviously the DC voltage here will be proportional to the signal strength.  The diode load voltage can go quite high on strong signals; of course it's limited by the AVC action.  If the AVC is off it can swing to -60V or perhaps more.

It's always a good idea to isolate the radio from an external amp with a cap.  You never know what kind of input circuit an amp has, and you certainly don't want to present a DC load on the diode load output, as it's a high impedance circuit.

This would be true of just about any radio.  That's why there's a coupling cap between the detector and the first audio; you don't need the rectified carrier voltage getting into the grid of the first audio as it will dramatically and dynamically change the operating characteristics of the tube.

In any event you don't need a non-polarized cap to couple it to your audio amp.  A 10 uF electrolytic with the "-" end going to the diode load will work fine.  If you're feeding into a high impedance amp you can reduce the value to 1 or even .1 uF.  The reason for the resistor on the other end of the cap is to provide a discharge path for the cap.  If the amp has a capacitor in series with the input the cap won't have anywhere to dischage itself, and it's gonna cause a huge "thump" if you connect/disconnect it.

Hi John,

That makes very good sense, and you are correct with reference to the R-390 and R-390A.

You make a very good point; when I first decided to take the audio off of the diode load, I measured the DC voltage at that point, to make sure it was indeed zero. In retrospect, I measured the DC voltage with no significant RF input to the receiver. Interestingly, however, I do not use a DC blocking cap between the diode load output of any of the receivers I had mentioned and my audio preamplifier, and I know of no ill effect to my equipment from doing this.

I wish to apologize to the Forum for diseminating incorrect information on this particular topic. As I recently had stated, a little bit of knowledge can be a dangerous thing.

Thank you again, John!

73,

Bruce

[w3jn]

In 99% of the cases it probably won't cause any damage.  But then you get someone who's designed an amp with no input blocking cap and a low impedance input.  Or a transformer input.  Another example might be one of those cheapo amplified computer speakers, not sure if the chinese bother to spend an extra 10 cents for an input cap.

Anyway it's easy to find out what your particular amp does with its input with a VOM.  Just be sure it can handle 60 volts or so.

[ke7trp]

I think Tom mentioned he is not using the cap and resistor. Just straight to the terminals. 

I use the cap and resistor. I used an 8UF since I had NO non polarized caps. Rat shack had a Drawer with 8s so thats what I used.

C