low level hum

[Paul Birkeland]

I would just get a 1/4" to 1/8" headphone adapter and leave that in while you're using speakers.  Assuming you're still on the 4 ohm OT configuration, having 10 ohms of resistance in parallel with whatever speakers you're using shouldn't be the end of the world, but you'll need the adapter in the jack to be sure it's 10 ohms and not 2!

[ccmccull]

Ah you're right, I hadn't thought of the effect of opening the jack switch. I am using the 4 ohm OT config. My speakers are 8 ohms so if I have my calculations right that makes a total impedance of 1/(1/8 + 1/10) = 80/18 = 4.4 ohms.

(And just for fun I tried unplugging the headphone adapter for a few seconds, which brings it down to 1.6 ohms. And indeed the sound output from the speakers got quite a bit softer.)

Thanks again.

[ccmccull]

Hi Paul,

Something occurred to me about this mod which concerns me, perhaps I have it wrong. If I disconnect the speakers and unplug the headphone adapter, then turn the volume all the way up, I'm sending the full output of the amp through the 2 ohm resistor which is only rated for 1/8 watt. I can easily go over 1/8 watt under reasonable amp input voltages. 

Of course I wouldn't intentionally do this, but it seems plausible to get into this state by accident.

What do you think?

Thanks!

[Paul Birkeland]

Yes, this mod is somewhat incompatible with speaker use. The series resistors that go across the jack don't cause this issue, but leaving the 1/4" to 1/8" adapter plugged into the jack is a decent compromise.

As always there is no free lunch!

[ccmccull]

Thanks Paul for your fast reply!

Let me clarify - I wasn't concerned about the compatibility of this mod with speaker use. (In fact my speakers work great even with this mod in place - I wrote a different thread on that topic.) My point is that even if I only use this as a headphone amp, I can get into a state where I'm pushing a lot of power through that 2 ohm resistor, much more than it's rated for. What I wanted to know is whether this is dangerous or not. I'm not particularly worried about myself, since I understand to never unplug the 1/4" adapter. But if other people have access to the amp, they might make this mistake.  Is my safety concern unfounded?

[Paul Birkeland]

The worst thing that will happen is the resistors will cook and open up.  As you said this would require leaving the amp on with no adapter and the volume pot up pretty far.  If you wanted some protection from this, you could plug in the 1/4" adapter and carefully paint some clear nail polish on the portion of the 1/4" jack contacts that short closed when you unplug the headphones.  This will leave the 8 ohm resistors in the circuit even without the adapter in place.  You may also be able to use a few small squares of electrical tape to the same effect.

[ccmccull]

That's a great idea Paul.

I built one of these for a friend and put in this mod because she uses Grados as well. (She loves it.) I think I'll ask her to bring it in for a service appointment!