SE Amps with Variable Impedance Loads

[antiquekid3]

Hey guys,

I am in the process of designing a SE 807 amp. In triode mode, I expect approximately 3.4W. I have already ordered Edcor GXSE15-8-5K OTs for this project. I wanted a low power amp for my newly-restored Klipsch Heresy speakers. However, my research tells me that the Heresy speakers vary their impedance from 10 ohms up to 70 ohms or so. Since transformers reflect the impedance of the secondary load to the primary, I can deduce that these speakers would NOT work well with a single-ended amplifier. Is this correct?

If that's the case, how much of a difference would it make if I put a 15 ohm resistor on the outputs? That should keep the impedance within what I would consider an acceptable load. What will this resistor do to the frequency response, and do you think I'd notice an appreciable drop in volume?

Thanks!

Kyle

[toprepairman]

The 15 ohm resitor would certainly help and whilst it would take a bit of power it won't be noticable.
I'd be inclined to include the transformer secondary in the cathode circuit. If you have a cathode bypass cap then instead of connecting it to ground, connect it to the speaker output (with the other end to ground). you will need to experiment as to which way round one of the windings is connected (pri or sec). one way will give you lower gain and smoother respose, and the other will give more gain, and may even oscillate.

[leadbelly]

Originally Posted by antiquekid3

However, my research tells me that the Heresy speakers vary their impedance from 10 ohms up to 70 ohms or so. Since transformers reflect the impedance of the secondary load to the primary, I can deduce that these speakers would NOT work well with a single-ended amplifier. Is this correct?

Not only is is not correct, but if your impedance dips to no less than 10 ohms, I would consider this a very tube-amp-friendly speaker. If you read up a bit more you will see that you are jousting at windmills, that's why the commonly quoted speaker impedance of 4,6, or 8 ohms is properly referred to as nominal impedance.

Originally Posted by antiquekid3

If that's the case, how much of a difference would it make if I put a 15 ohm esistor on the outputs? That should keep the impedance within what I would consider an acceptable load. What will this resistor do to the frequency response, and do you think I'd notice an appreciable drop in volume?

Forget the resistor, you don't need it, it's a bad idea.

[antiquekid3]

I haven't really heard of putting the secondary in the cathode circuit before. That's definitely an interesting idea.

I got my info from here, and the general consensus was that the Heresy was not SE-friendly. http://community.klipsch.com/forums/p/77869/771253.aspx

Also, many people say never to run a tube amp into no load, which is infinite impedance, technically. Since 70 ohms is much higher than 8 ohms, I would guess that it's not very good for it. That'd be reflecting 43.8KΩ into the primary, I believe.

Kyle

[artosalo]

However, my research tells me that the Heresy speakers vary their impedance from 10 ohms up to 70 ohms or so.

This is not an unusual impedance behaviour. Rather it is quite typical.
The impedance of a typical speaker is relatively high at bass resonant frequency.
Fortunately there is a cure for this. The proper amount of negative feedback. This reduces the negative effects of varying load impedance.

I can deduce that these speakers would NOT work well with a single-ended amplifier. Is this correct?

It is not correct. Same problem exist with pp-amplifiers.
The worst results will exist with pentode-connected amplifier. With and without feedback.
Best results exist with UL-connected amplifier and little worse with triode-connected. Also with and without feedback.

I have already ordered Edcor GXSE15-8-5K OTs for this project.

If I were doing such amplifier, I would use UL-connection since this transformer makes it possible. Then you will get some 9 watts with same distortion level that 3,4 watts from triode stage.

[antiquekid3]

My current design is looking like either a 6SL7 single-ended front end or a 6SL7 SRPP, both possibly with an LED bias, just for fun. Right now I don't have any feedback (that I know of). If I did use negative feedback, what would I need to do and how would I calculate the values?

If the higher impedance really isn't a problem, then why do people say not to run a tube amp into no load? What will happen?

I plan on having a switch to switch between UL and triode modes. I'm working on my schematic(s) right now, so we'll see how it goes.

Kyle

[DF96]

A 15R resistor would waste 40% of your output power. Not a good idea.

Triode or UL should be OK. They will help damp the bass resonance. As you are going for SE I assume you don't want a low distortion design with global feedback.

No load is a problem, especially at higher frequencies. A higher resistance is not a problem, especially at lower frequencies. If you want to use feedback then you either need to learn a bit more or copy an existing design, as feedback can create as many problems as it solves if not done carefully.

[atmasphere]

The 70 ohm impedance will not be a problem. That is at a resonant frequency and obviously your amp will not make much power into that load.

But it won't have to, nor would you want it too, as if the amp made the same power, it would be *way* too pronounced at that frequency. If I were you, I'd give it a shot- certainly it will not be hard on the amp

[antiquekid3]

Okay, great. I first have to get the amp built, and then get some tweeter diaphragms for the K-77s.

Thanks!

Kyle

[artosalo]

That is at a resonant frequency and obviously your amp will not make much power into that load.

Without negative feedback the amplifier will obviously be overdriven at this frequency and produce severe harmonics.

I first have to get the amp built, and then get some tweeter diaphragms

If you later would like to add the negative feedback take care that your amplifier has sufficient amount of extra gain. Otherwise this is impossible to do.