Hi,
I've looked around and asked this question in various threads but still haven't fully understood this. The context of the question is that I'm trying to build a 45 based SE headphone amp. Let's assume a standard 5K:8R transfomer is used, and that it would have 2W available to 8ohm speakers. Now, what happens when 32ohm and 300ohm headphones are connected to the 8ohm tap?
This is my understanding:
If headphones are connected directly, 32ohm will have 500mW and 300ohm will have 50mW. The reflected load on the primaries should ideally be around 5K. When connected directly, the load will be 20K and 187K respectively.
OTOH, connecting a 12ohm/3W resistor across the output would "fake" the reflected load to 5.4K-7.2K, which seems better. In this case though, how much power is being "wasted" on the load resistor and how much remains for the HP's?
I've basically gotten conflicting answers on whether the load resistor is required or not, hence my confusion.
Lastly, I would like to understand what are the differences in Voltage swing for the headphones, if they were connected to an 8ohm tap vs. a 32ohm tap.
Thanks!
I've looked around and asked this question in various threads but still haven't fully understood this. The context of the question is that I'm trying to build a 45 based SE headphone amp. Let's assume a standard 5K:8R transfomer is used, and that it would have 2W available to 8ohm speakers. Now, what happens when 32ohm and 300ohm headphones are connected to the 8ohm tap?
This is my understanding:
If headphones are connected directly, 32ohm will have 500mW and 300ohm will have 50mW. The reflected load on the primaries should ideally be around 5K. When connected directly, the load will be 20K and 187K respectively.
OTOH, connecting a 12ohm/3W resistor across the output would "fake" the reflected load to 5.4K-7.2K, which seems better. In this case though, how much power is being "wasted" on the load resistor and how much remains for the HP's?
I've basically gotten conflicting answers on whether the load resistor is required or not, hence my confusion.
Lastly, I would like to understand what are the differences in Voltage swing for the headphones, if they were connected to an 8ohm tap vs. a 32ohm tap.
Thanks!
Can you post the circuit? A resistor in parallel with the load will split the power according to 1/R for each, since power = V^2/R, and V is the same for both.
Last edited:
The circuit is a Tubelab SE. I don't think there is any NFB.
OK so assuming the same 8ohm output, 32ohm HP - 12ohm load resistor gets ~72% of power and 32ohm gets 27%, HP will have 545mW. 300ohm HP gets ~3.8% which is 76mW.
If these calculations are correct, then overall power to the HP's seems the same with or without the load resistor, but the reflected load on the triode is different in both cases. Without load resistor, the tube is being "under" worked, while with the load resistor, it's being loaded to full power.
Is this correct? If so, which is the preferred scenario? There has to be a "correct" way to do it.
OK so assuming the same 8ohm output, 32ohm HP - 12ohm load resistor gets ~72% of power and 32ohm gets 27%, HP will have 545mW. 300ohm HP gets ~3.8% which is 76mW.
If these calculations are correct, then overall power to the HP's seems the same with or without the load resistor, but the reflected load on the triode is different in both cases. Without load resistor, the tube is being "under" worked, while with the load resistor, it's being loaded to full power.
Is this correct? If so, which is the preferred scenario? There has to be a "correct" way to do it.
George and another source recommended the resistor approach and 2 other sources mentioned that loading with the resistor is not necessarily required. I'm trying to understand the theory behind both approaches.
It seems that in terms of overall power it doesnt make much difference, but there is a big difference in terms of the load on the triode. I simply don't know what are the tradeoffs of each approach
George designed the circuit, so you should best go with his approach. Changing the load moves the load line around, and may limit the available voltage swing, increase distortion, and/or be asymmetrical. Most output transformers are optimized for a particular impedance, and should be used with this in mind.
The power is not an issue, since headphones need little power. Also, a parallel resistor will tend to flatten the impedance curve seen by the amplifier and make it more resistive, which is a good thing, especially for a circuit with highish output impedance and no loop nfb. The voltage output with frequency will then be more flat with a flatter impedance curve.
The power is not an issue, since headphones need little power. Also, a parallel resistor will tend to flatten the impedance curve seen by the amplifier and make it more resistive, which is a good thing, especially for a circuit with highish output impedance and no loop nfb. The voltage output with frequency will then be more flat with a flatter impedance curve.
Last edited:
Thank you, this makes sense.
So, assuming a parallel resistor is used, what would be the difference between using an OPT that is tapped at 8ohm and one with 32ohm? With the 8ohm tap, a 10-12 resistor would be used to match impedance. ~47 ohm would be used with a 32ohm tap.
Power output would be the same in both cases, right? But, with the 32ohm tap there would be a higher voltage swing. Is this correct?
So, assuming a parallel resistor is used, what would be the difference between using an OPT that is tapped at 8ohm and one with 32ohm? With the 8ohm tap, a 10-12 resistor would be used to match impedance. ~47 ohm would be used with a 32ohm tap.
Power output would be the same in both cases, right? But, with the 32ohm tap there would be a higher voltage swing. Is this correct?
Right on both. I suspect that you will prefer the sound one way or the other though, so it try both ways. Using the lower impedance tap (and lower resistor) will result in a flatter impedance curve for a given headphone. The higher impedance tap may have different advantages, like better hf.
Last edited:
Thanks again!
I still haven't ordered the transformer.
The amp will primarily used with high impedance HP's which benefit from a high voltage swing, so I think I'll go ahead with a single 32ohm secondary.
Lastly, is there a simplified way to calculate the available voltage? Is it as simple as dividing the output voltage from the triode by the transformer ratio?
I still haven't ordered the transformer.
The amp will primarily used with high impedance HP's which benefit from a high voltage swing, so I think I'll go ahead with a single 32ohm secondary.
Lastly, is there a simplified way to calculate the available voltage? Is it as simple as dividing the output voltage from the triode by the transformer ratio?
If you know the rated output power into a particular load R, the power P = (Vrms^2) / R , and then Vrms = sqrt (P x R).
If output power is 8W into a load R of 32 ohms, then the rms output voltage is sqrt (8 x 32) = 16 Vrms.
Last edited:
The output power is the same in both cases, so if P = 8W = V^2 / R,
then for an 8 ohm load, 8W = V^2 / 8, and V = sqrt (8 x 8) = 8Vrms.
Also for a 32 ohm load 8W = V^2 / 32, and V = sqrt (8 x 32) = 16Vrms.
I think that 8W into any headphone should be more than plenty.
Last edited:
The 8W must be for a different tube in that circuit.
If 2W = V^2 / 8, then V = sqrt (2 x 8) = 4Vrms for an 8 ohm load.
If 2W = V^2 / 32, then V = sqrt (2 x 32) = 8Vrms for a 32 ohm load.
The 2W is still likely to be an order of magnitude larger than you'd ever actually use.
Last edited:
Keep in mind that there really is no " mis-matching" or "matching: with respect to loading a triode... Matching is for transmission line applications... The tube does not give a S##T what load you put on it...as long as the load swings in a safe region over the average of a cycle...
When a particular transformer is at a specific plate load, usually for minimal distortion ....then the inductance, leakage and winding capacitance will determine the bandwidth for that given plate load.... Once the reflected load changes, then the bandwidth will move .... That is usually why you stick with the intended plate load of a transformer...unless you intentionally change the reflected load and have re-calculated the new bandwidth to be acceptable..
When a particular transformer is at a specific plate load, usually for minimal distortion ....then the inductance, leakage and winding capacitance will determine the bandwidth for that given plate load.... Once the reflected load changes, then the bandwidth will move .... That is usually why you stick with the intended plate load of a transformer...unless you intentionally change the reflected load and have re-calculated the new bandwidth to be acceptable..
The main thing with a valve amp is not have an open circuit output.
Even loading a 8R output with a 220R resistor should prevent any problems.
So 300R headphones should be fine and so is 8 ohms.
Sorry but I'm missing something in your explanation here. You're saying that it doesn't matter if the reflected load on the tube is 5K or 100K? I forgot where I saw it but one resource mentioned that ideal load was 3*Rp (which is around 5K for the 45), and in another, there was a graph that showed the distortion/power at a given impedance (I couldn't find such a graph for the 45). Based on the graph, there was an "ideal" load to put on the tube for lowest distortion.
Or is that what you are saying?
That makes sense. How do speaker amplifiers protect themselves? Can those without a dummy load get damaged if turned on without speakers?
Tube amps can usually *idle* without a load. OVER-drive, clipping, without a load causes large kick-backs which damage transformer insulation.
Best power in a transformer coupled triode is often near 2*rp (depending on ratings. Best distortion is at *infinite* load, but that is zero power. Do not worry about loading in 10X or 20X rp if you get enough power output.
Best power in a transformer coupled triode is often near 2*rp (depending on ratings. Best distortion is at *infinite* load, but that is zero power. Do not worry about loading in 10X or 20X rp if you get enough power output.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.