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Old 13th August 2012, 05:23 PM   #11
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Ahhhh I see, yes these are for home stereo use though hopefully they sound like they are producing music by the time I'm done. I'll give that all a try when I get back home tonight and see what happens, thanks for the help.
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Old 13th August 2012, 06:01 PM   #12
ChrisA is offline ChrisA  United States
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Originally Posted by cthcamaro67 View Post
So I've run into quite a conundrum here. I've just bought two Northern Electric 4045-C Amplifiers. Powering either one on by itself the amplifier sounds great, little bit of hum but other than that great. However, if I turn them both on at the same time (they are monoblocks so I do need to have them both on for stereo) they start to distort horribly.
Logic says that one monoblock needs to some how "know" the other one is on. This means there must be a communication path between the two units.

There are tree possible paths and you have to address each

(1) Electric or magnetic field coupling. Try placing the two units 10 or 12 feet a part and see if the problem goes away. The transformers might couple if the units are close

(2) Both units are connected via the AC mains power. Try using two LONG extension cords and pluging each into a different outlet. Hopefully on a different circuit. Use a EMI filter "spike" filter on each unit near the outlet

(3) grounds. This is my bet. Are these amps must be fed from a stereo preamp? If so maybe you have something like a ground loop. Did you say they used a balanced input. If it is not transformer coupled you might have problems. try reversing the polarity of one of the inputs. Look for a ground loop. Music store sell ground lift devices you might try a few of those too. A place like Sweetwater or Sam Ash or any pro-audio place. I'd bet on a ground loop or common mode problem. But do rule out #1 and #2 above.


Again the way to think about this is that there is a "communication path"
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Old 13th August 2012, 06:08 PM   #13
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Oh man I'm liking the transformer proximity theory, they are right next to each other and quite far apart when I have them hooked up in my friends shop. I'll certainly try plugging one into another circuit as well.

As for how they are hooked up it gets a little complicated. From my preamp the output goes into a McCurdy Am-405 which is transformer coupled (in and out). I then take the speaker out from this and convert it to RCA male ends on the other side, which is how I feed the signal to the Northerns. They don't have enough gain otherwise nor do they have an output stage so the McCurdy solves that problem. So the McCurdy acts as and interstage or driver amplifier. I can't help but think there is a better way to connect the two though.
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Old 13th August 2012, 07:22 PM   #14
ChrisA is offline ChrisA  United States
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Originally Posted by kevinkr View Post
My understanding is these are used as part of your stereo system and are not a guitar or instrument amplifier so it may be legitimately be posted to Tubes / Valves. Instruments and Amplifiers is intended for devices that make music not reproduce it.

Grab an AM/FM radio and turn it on, tune across both bands while the amps are misbehaving and see if you hear anything.. Be quick of course so as not to damage anything - hopefully all will be silent - no squeals or odd noises due to amplifier oscillations.

Make sure the AC connections to both amplifiers are wired the same way in the event that leakage current is the cause of this issue. I'd recommend in any event grounding the chassis of both amps for safety..
So you are using a 30 watt tube amp as a line level booster? Are you using a dummy load to simulate the miss speaker? the NFB loop in the amp assumes the voltage one would get when driving a low impedance load, like a speaker.

And RCA jacks for balanced signals? I'm things that maybe some how there is a DC path through the input transformers and they saturate.

Can you draw and end-to-end schematic that shows all the grounding connections and transformers? Without that we are all just guessing
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Old 13th August 2012, 08:12 PM   #15
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I'll see if I can work an end-to-end schematic when I get some time. My preamp has 6uF output caps so I can also run that straight into the Northerns and omit the McCurdy all together, loss of gain but 12oclock on the preamp drives them just fine. Still only one amplifier will work at at time, turning both on creates the huge pink noise distortion. Almost like an RCA has been unplugged. I'm not using a dummy load, I was told the McCurdy was originally intended for this type of application (a friend of mine has been running it like this for quite some time with great results).
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Old 14th August 2012, 02:32 AM   #16
kevinkr is offline kevinkr  United States
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The McCurdy 405 seems to have been designed for these sorts of applications, output power apparently is not that high - does it have 600 ohm outputs?

I suspect the right way to connect these is probably balanced using XLRs with no ground reference between the 405 and NE power amps.

Magnetic coupling is quite possible if the amplifiers are close, OTOH my GM70s are transformer coupled and right next to each other..

What is different between your friend's shop and your house? Power? Source driving the amps? 1Megawatt microwave transmitter just outside your livingroom.. Just kidding..

Joking aside have a look at what he is doing that you are not.. Keep us posted.

And please put up some good pix of the McCurdy 405 along with details about output connections available.
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Old 14th August 2012, 03:44 AM   #17
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I'm pretty sure that these are more booster line amps than a conventional power amp they have 600 ohm line input transformers and will take a high db level to drive them .

Also with a 600 ohm input it will put a lot of current load on what ever is driving it , if you using a preamp or CD player with an op amp output it may be over heating when both amps are connected , if a dual package op amp woun't stand both operating at the ame time themaly.
Try to mono preamps or if you have a couple matching transformers like UTC A-25's between the signal scource and the amps may help .
Also i went back and looked at the schematic for the power supply and it was made for electro dynamic speakers , so adding just a resistor to replace it will cause hum problems , you would need a choke resistor and cap to do it right .
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Last edited by battradio; 14th August 2012 at 03:50 AM.
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Old 14th August 2012, 04:07 PM   #18
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Well I managed to move my industrial 1 megawatt microwave transmitter off my rack so I'll have to put my efforts of other-worldly contact aside for the sake of audio, just for the time being tho . In all honesty I managed to solve the distortion problem and as it turns out my friend has much better cable management skills than myself. Five or so interconnects snaked through some power cords and speaker cables did not lend itself well to a distortion free environment. Just got the rack so I haven't quite spent time setting it up properly. Transformers also had to be moved away from each other to take out the last bit of low level hum (last pic shows that they didn't have to be moved that far).

battradio, my transmission line is as follows, sonos -> matrix dac -> preamp -> McCurdy (8ohm speaker out) -> Northerns -> Speakers. My preamp is tube based (6SN7) and I've included a picture of it. All the wiring is under the board otherwise I'd snap a good pic of that as well. It has 6uF paper and oil output caps so it drives the transformer input with ease. DAC is the only thing with op amps and they are only used as buffers at that.

I included a few pictures of pretty OK quality, if I had been reading this thread last night I would have snapped some new ones. Let me know if these are dreadfully unacceptable and I'll snap a few more for you Kevin. First pic is of the internals of the McCurdy, it's not a schematic by any means but it should give you a good idea of what's going on. A different friend of mine was an old Sears tech and restores vintage Western, IPC, Motiograph etc so he rewired the McCurdy like you see it now. Not sure how it was originally wired. I'd ask him for help if he wasn't notoriously difficult to get a hold of. The second picture displays the transmission line I described above. Third pic is quite unacceptable but it is the only one I currently have, top shot of my preamp, wiring is underneath the board so I cannot quite get a picture of that. The grey rectangles are vintage Russian paper and oil caps. Fourth shot is of the Northerns, they are half and inch to short for the rack so they are on the floor till I bolt on an "ear extender." You can see out of the closest octal plug a protruding wire, on which I soldered my RCA inputs.

So I guess my question now, is there a better way to "slave" the northern amps to the McCurdy? I find it hard to believe this is how they would have originally been set up. The McCurdy has a gain knob for each amp and I've reduced it by about a quarter turn as the gain is quite high when they are mated to the Northerns. Their original connections (in and out) were the octal plugs I mentioned earlier. Anyway thanks again for the help, it is much appreciated!
Attached Images
File Type: jpg McCurdy inside.jpg (688.6 KB, 44 views)
File Type: jpg trans line.jpg (572.9 KB, 41 views)
File Type: jpg Preamp.JPG (496.4 KB, 41 views)
File Type: jpg Northerns.jpg (795.3 KB, 43 views)
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Old 14th August 2012, 04:43 PM   #19
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Hi cthcamaro67, the pix look pretty good and detailed. The getter on one of your pre-amp tubes looks close to gone - can't tell what type it is. (6SN7??)

Just wondering now whether the outputs of the McCurdy are floating or grounded with feedback - it appears that there is no feedback connection..

If the outputs are indeed floating a good move would be to use twisted pair to the Northern amps, and depending on the load presented by the NE amps it might be good to place some resistive loading across the output of the McCurdy amps. What are the available output impedance connections on the 405s, looks to me like 0, 4, 8, 16 (I have no clue) - the 4 ohm tap would give you 6dB less output voltage than the 16 ohm tap reducing the gain by the same amount.
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Old 14th August 2012, 05:11 PM   #20
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Yeah that was an old shot and I have since replaced the tubes, 6SN7 it is. The way it is wired externally the amplifier has 4 and 8 ohm outputs. Floating or grounded with feedback, that is a tad beyond my skill level. Would you mind explaining how I could figure this out? The way I have it setup it certainly works and sounds spectacular, is there any way I could be harming anything?
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