FET ten/hl recorder output puzzle - diyAudio
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Old 17th October 2014, 12:11 AM   #1
Lavcat is offline Lavcat  United States
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Default FET ten/hl recorder output puzzle

I have a puzzle. My plan was to connect a set of Bose powered computer speakers to the #2 recorder output of the FET ten/hl, using a Jensen iso-max between the recorder output and the speakers.

When I did this a small but noticeable amount of hum and noise was present at the main output of the FET ten/hl, which increased or decreased with the volume control. There was also unusually high hum and noise in the speakers. Powering the speakers off made the hum and noise go away.

I tried a couple different sets of cables between the FET ten/hl and the iso-max, and I tried plugging the speakers (which are double insulated) into different power circuits. No change.

I replaced the iso-max with a pair of Radio Shack female to female RCA adapters, and the system was again dead quite. And the speakers worked.

Feeling a little stupid by this time, I reconnected the iso-max and tried moving the position of the box relative to the speaker unit. No change, still the same noise. If the noise was being magnetically coupled I would have expected some difference with position. Plus another iso-max and other Jensen transformers in close proximity were picking up no noise.

In the past I've hooked up the FET ten/hl recorder outputs to tape recorders and I don't remember any problems.

I should add that the iso-max works normally on FET ten/hl inputs. Wonderfully, even. A night and day difference in some cases. On the recorder output there was a night and day difference, just not such a wonderful one.

Can anyone help me figure out what is going on? I have not yet tried swapping out different iso-maxes (or whatever the plural of iso-max is called), nor did I test with recorder output #1.
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Old 19th October 2014, 11:48 PM   #2
Lavcat is offline Lavcat  United States
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A few more experiments:

I tried another iso-max.

I tried recorder output #1 rather than #2.

I disconnected all other outputs and inputs from the FET ten/hl.

None of these made any difference in the hum and noise. Then for a sanity check I connected everything back up with an iso-max connected to a recorder output, but with the output of the iso-max not connected to the powered speakers: no noise.

My computer sound card directly connected to the powered speakers through an iso-max (in other words not using the FET ten/hl) works without hum or noise.


Any other experiments that it would be worthwhile to try? Is there anything unusual about the FET ten/hl recorder outputs?
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Old 20th October 2014, 04:17 PM   #3
GaryB is offline GaryB  United States
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Quote:
Originally Posted by Lavcat View Post
Is there anything unusual about the FET ten/hl recorder outputs?
It seems pretty clear that there isn't a problem with the FET 10/hl. It's a problem that you're creating when trying to hook up the Bose powered speakers using a transformer. The Bose speakers probably have no internal ground connection and when you let them float through the Isomax, they inject 60hz into the preamp. You say things are dead quiet when you get rid of the Isomax and hook up the speakers directly to the FET 10/hl. So just do that.
---Gary
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Old 20th October 2014, 06:06 PM   #4
llwhtt is offline llwhtt  United States
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Bose and Threshold in the same sentence, that isn't right. Should be a law against stuff like that.

Craig
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Old 20th October 2014, 08:47 PM   #5
Lavcat is offline Lavcat  United States
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Quote:
Originally Posted by GaryB View Post
It seems pretty clear that there isn't a problem with the FET 10/hl. It's a problem that you're creating when trying to hook up the Bose powered speakers using a transformer. The Bose speakers probably have no internal ground connection and when you let them float through the Isomax, they inject 60hz into the preamp. You say things are dead quiet when you get rid of the Isomax and hook up the speakers directly to the FET 10/hl. So just do that.
---Gary
Thanks for the response. I'm not sure the theory accounts for all the facts. The CD player, the computer sound card, and the main outputs of the FET ten/hl all work well driving the Bose speakers through an iso-max. Only the recorder outputs have the problem.

Directly connecting the Bose speakers to the recorder outputs works pretty well. There is no noise at the listening position, though there is some with my ear to the speaker, noise that was not there with the main output of the FET ten/hl connected to the speakers through an iso-max. I tried the main outputs with and without an iso-max and the iso-max made a positive difference. "Dead" is relative, I suppose.

I am still puzzled what is causing the hum problem.

As another experiment I connected the FET ten/hl recorder outputs to a Crown D-75A through Jensen JT-6110K-B transformers. Not an iso-max but same idea. There was noise that is not there with the main outputs of the FET ten/hl connected to the Crown D-75A through the Jensen JT-6110K-B transformers. I believe this shows the fault is not with Bose.
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