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SE EL84 amp help - grounding output terminals?


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I've been a little stuck lately trying to get the REL sub working (without emitting an alarmingly loud buzzzzz!) with this otherwise excellent tube amp. The issue has been identified as the floating output terminal on the amp not providing a grounded connection for REL's hi-level connection. As the REL tech replied:

 

Quote

Justin Magana (REL Acoustics)

Jul 9, 2020, 3:49 PM PDT



I have looked at a few of the Decware designs and they either float the negative speaker output terminal, or connect it to ground through a 4.7 ohm 5W resistor.
The latter design should work without hum, but the former (floating design) will cause the REL to hum.

I would try to find out from the designer definitively which design is employed and if you could either ground the negative output with the signal input ground (chassis) or if the  4.7 ohm resistors can be added.

The other possibility is that there may be a problem with the REL.
Try this:  with the REL on and the high level cable plugged into the input, take the three bare wires (RED, YELLOW. BLACK) and combine them together.  If the REL hums, or there is no change in hum when the wires are combined then there is likely a (fixable) problem with the REL.  If the hum goes away, then the REL is fine.

FYI, since this amp is a dual single ended design, you may hear a slight hum when it's working because it is difficult to reduce power supply noise unlike a push-pull design.

 

Best regards,
Justin
Chief Engineer

 

Now I can solder... but I can't interpret circuit diagrams and have no electronics background. But I know there are many SNA'ers who have both of these :D 

 

Can someone please help with the "ground the negative output with the signal input ground (chassis) or if the  4.7 ohm resistors can be added" part?

 

I have attached a pic of the underside of the amp in question, as well as the schematic it was based on. Better pics or video are certainly available :thumb:

 

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Your amplifier says it's specificlly designed with floating outputs and "not to do it".   You should ask Decware....  as they will surely be able to explain to you.

 

If you do, it won't be the chassis that you will use..... but you may be able to tie the speaker output to groud with a large resistor (even though the indicate don't) ......   but I'm not sure, it might wreck the performance of the amplifier.

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In the old days chassis used to be occasionally used for ground...have a cap that goes to ground then just solder it to the chassis.  Not the best arrangement.  Looks like your amp uses a dedicated ground line as all well designed point-to-point amplifiers should, so that it good.

 

The secondaries of the single ended output transformers should be able to be connected to your ground point/buss with or without a resistor because by nature the output of the transformer is already "floating".  The wire to ground gives it a "reference", or tells the speaker wires what potential the common should have by tying it to that of the entire amp, or floating it a little if using a resistor.  It should not be an issue with your amp to do this, but I would certainly contact Decware if possible and talk it through with them.  Sounds like you would oly have to reference output transformer secondary that you use for the REL sub. 

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@crtexcnndrm99Ok, just read through the Decware "instructions" you included earlier and it seems the only reason not to ground the secondaries of the OPT's is that you will not be able to connect the two OPT's in series if you want to run the amp in mono i.e. double the output power and then get another amp for the other speaker.  If you don't do this (which it appears you do not) then go ahead and make the ground connection, because it will not hurt anything.  You can try it first with one OPT (the one for the REL sub) using an alligator lead.

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  • 2 weeks later...

Update:

 

Removed tubes and turned the amp upside down today. Connected alligator clips from the black speaker terminal of each channel to the ground connected to the input RCAs. Added in the REL subwoofer at hi-level... no hum :cool:

 

Unless they fall off, the alligator leads will probably stay there for now, as an easily removable solution. 

 

First time listening to the new DIy speakers with the sub. I predict a long listening session tonight....

 

 

I appreciate the input and advice offered above - thank you!

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In case this is helpful:

 

I have connected my REL subs (I have owned several models) using high level inputs to many kinds of amplifier over the years. Even had the special Synergistic cable. Several years ago I was using a Wyred4Sound ST-1000 stereo amp and there was talk of Class D and high level connection not liking each other....I then found the low level to be quite OK. First time for low in 25 years!

 

In the end I now always utilise the LFE leads from the preamp to the REL because it crosses over at 30Hz by default (I find this value ideal in my system) and because my preamp has  sub out terminals. I could not tell the sonic difference between the high and the low level connection. Really.

 

The LFE is very convenient. I know what REL always suggests (high) but everything works and sounds very good with LFE.

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18 minutes ago, doogie44 said:

In case this is helpful:

 

I have connected my REL subs (I have owned several models) using high level inputs to many kinds of amplifier over the years. Even had the special Synergistic cable. Several years ago I was using a Wyred4Sound ST-1000 stereo amp and there was talk of Class D and high level connection not liking each other....I then found the low level to be quite OK. First time for low in 25 years!

 

In the end I now always utilise the LFE leads from the preamp to the REL because it crosses over at 30Hz by default (I find this value ideal in my system) and because my preamp has  sub out terminals. I could not tell the sonic difference between the high and the low level connection. Really.

 

The LFE is very convenient. I know what REL always suggests (high) but everything works and sounds very good with LFE.

No it is helpful, and I don't think you're the only one to go this way with the RELs. 

 

At present I don't have a preamp, but will be revisiting LFE when I do. 

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