UglyJoe
100+ Head-Fier
- Joined
- Dec 11, 2006
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The idea for a “new” SET transformer-coupled headphone amplifier has been rolling around in my head for a while now, and is the result of several factors.
1.)There seems to be a general lack of vetted and community-wide SET designs out there. That’s not to say that you can’t find them, but there aren’t any current designs that have the kind of user base that say a beta22, M^3, SOHA, Bijou, etc., where new builders can easily post in a current thread and quickly get help with there issues etc. Also, I haven’t seen a lot in the way of design ideas being spread around in a long thread with comments and criticism from many designers, which I think in the long run ends up producing a better amplifier.
2.)A lot of the designs that are out there if you scour the web tend to be less than user friendly, particularly with regard to using rare parts, using transformers that aren’t intended to be output transformers, or using custom wound transformers. Although the custom made route is probably the best in this case, I wanted to use something “off the shelf”, simply because I feel more people are likely to buy and experiment with an available product than to go through the hassle of getting something custom made, even if the price difference isn’t all that large.
3.)It’s been my (unconfirmed) guess that many of the commercial SET designs out there use a resistor in parallel with the load on the secondary of the output transformer. This “trick” allows the amp to drive headphones of multiple impedances without having to use different secondary taps. The headphones are in parallel with a low-ohm resistor (typically 4-8R). Because most headphones are much higher impedance than the resistor, the overall impedance of the load tends to be about that of the resistor alone, which means that headphones of any impedance can be used, as long as their impedance is typical (30-600R). The obvious drawback here is that a huge amount of power is wasted across the resistor. SET amplifiers are already notoriously inefficient as it is, and I simply find this resistor method to be an inelegant engineering solution. As such I wanted to use a transformer with secondary taps such that they could be matched to a specific headphone. Designing some kind of tapping scheme such that the amplifier could be used with more than one type of headphone is even better.
All of the above led me to attempt to put together something that is at least a start on a DIY SET transformer-coupled headphone amp. NOTE: I AM NOT AN EE. I am an enthusiast who has read a few books and is trying to piece something together that will sound good and operate safely and with good stability. As such I don’t expect this in its current form to be prefect, or anywhere close, to be honest. I am doing this as much as a learning experience for me as for anything else. I welcome ideas, criticism, etc., as long as they are constructive.
Project: A DIY SET Transformer-Coupled 2A3 Tube Amplifier
Goal: The design of a high quality transformer coupled headphone amp using current-production or easily obtainable parts, including the output and mains transformers. Because the iron will be expensive, no effort will be made to go cheap on the other components or the design, however, complexity at a high cost to performance ratio will be avoided. The amplifier should be easily configurable to drive “common” headphones…. 32R, 300R, etc. Being a SET amplifier, there won’t be a huge emphasis on low distortion, etc. However, there will be an emphasis on keeping distortion of the even-order variety, and where it is easy to minimize, steps will be taken to do so.
The first and most limiting choice is the output transformer. Because I wanted to use an off the shelf model, this limits the design choices in the output stage. Also, there simply aren’t that many OTs available for headphones. Many other designs I have seen have chosen to use interstage transformers, as there are more of them on the market. I have chosen to use one of Sowter’s headphone OTs. I decided this because they are designed for SET headphone amps, they have taps for many common headphone impedances, and because I haven’t seen them used in other designs I have come across, and it seems a shame to have a transformer on the shelf and ready to go and not used in common DIY designs.
For the output stage I chose to use a 2A3. This was based on a suggestion from another forum member. I’m not an experienced tube roller and didn’t really have a preference on the output tube; however, I’ve read great things on the 2A3 and as I said it was suggested to me by another member who is much more experienced than I am at this. There are current production models, and you can get a decent sounding matched set of Sovtek’s for 70-80$.
I designed the output stage around the suggested operating point of Va = 250V, Iq = 60mA, and Vgk = -45V. This choice required the Sowter 8976. The 8976 has a primary Z of 2k, with secondary Z configurable to 300, 150, 75, and 18 ohms. This is achieved with 4 equal secondary windings. Applying them in series gives a 2.5:1 turns ratio, while applying them in parallel gives a 10:1 turns ratio. Different combinations of series and parallel application allow matching the headphone impedance to the desired primary impedance. The output stage requires this transformer because it is the only off the shelf Sowter headphone transformer than can handle the quiescent anode current of 60mA.
I’m intending this amplifier to drive my 32R Grado HF-2s. As such with a 10:1 turns ratio the transformer presents a 3200R impedance to the output stage. Sennheisers can achieve nearly the same impedance by wiring two of the secondaries in parallel and then in series with the rest of the secondaries, giving a 3.3:1 turns ratio and a load impedance of 3333R. The plate characteristics of the output stage are presented below, with the loadline, maximum anode power, operating point, and maximum achievable voltage swing all marked (for 32R headphones).
I’ve chosen to use a resistor in the cathode circuit to bias the output stage and bypass it with a capacitor. I’m open to suggestions on other arrangements here, however I’m actually pretty comfortable with this method of biasing. The negative feedback at DC keeps the circuit stable and at intended operating parameters better than other biasing methods, and I don’t desire the use of a negative power rail to bias the grid directly. The required capacitor for a f-3dB is 42uF. The total voltage swing available is 273 Vpk-pk, which translates to a full ouput power of 2.9W into 32 R headphones. The predicted distortion of the output stage is approximately 5.5%, composed mostly of second order harmonics. This distortion falls linearly with output power, so at 1/10th the output power, distortion would be 0.5%, about typical of a SET amplifier. Gain of the stage is approximately 3 (before being reflected through the transformer).
The chosen operating point of the output stage is with a Vgk = -45. For maximum output swing, the stage must be driven by 32VRMS. The input stage must therefore have an input sensitivity of approximately 2VRMS (to be driven by the line-out of most DACs on the market today), a gain of at least 16, and an output impedance of less than 11k (this was calculated based on the Miller capacitance of the output stage and a f-3db of 150kHz).
I’m fairly pleased with the output stage. The driver stage is another matter. There are many choices available to use in this stage. For this first iteration, I’ve decided to start off simple. We can enhance the stage to make it better from here. For now, the design uses a 6DJ8, and is nearly stolen from Regal’s mod to runeight’s Bijou. I’ve modified the chosen operating point slightly to Va = 108 and Iq= 5mA. This requires a RL of 28k4 and a HT of +250 V and gives a Vgk of -3V. This was chosen based on the output voltage of my DAC (1.7VRMS). This voltage gives a maximum Vpk-pk of 4.8. I wasn’t able to find in any 6DJ8 datasheets at what Vgk the onset of grid current begins but I believe it is around -1V for most tubes. Under most operating conditions the input signal should remain at or below 4 Vpk-pk, so Vgk of -3 seemed to be a reasonable choice. At this operating point the calculated rp was 5k2. As is, the current stage has a gain of 28, approximately the correct input sensitivity, and an output impedance of 4.4k, all of which fall into our needs for driving the ouput stage.
Improvements should be made to the input stage before moving on with the design. Currently, the gain is really too high to drive the output stage, and the stage is noisy and will cause a large amount of distortion (comparatively). I think that this can all be improved with the use of some negative feedback and/or an active load for the stage. I’ve briefly considered a u-follower, B-follower, or SRPP stage in place of the current design, but haven’t really tackled those till I’m sure that the added complexity is really needed. Any suggestions would be greatly appreciated.
Here is a schematic for the amplifier in its current form. Note: I have sent a request to Sowter to inquire about the DC resistance of the primary of the 8976. This is required before the final value of the HT can be known. After I get this info I’ll post my thoughts on a power supply and its options. Also note I mislabeled the cathode voltage of the output stage. This should be 45V, not 50.
I don't have any of the software to simulate this circuit. I would appreciate anyone that does letting me know what it looks like. I'd also appreciate lots of input, particularly on improving the input stage, but any comments will be helpful.
1.)There seems to be a general lack of vetted and community-wide SET designs out there. That’s not to say that you can’t find them, but there aren’t any current designs that have the kind of user base that say a beta22, M^3, SOHA, Bijou, etc., where new builders can easily post in a current thread and quickly get help with there issues etc. Also, I haven’t seen a lot in the way of design ideas being spread around in a long thread with comments and criticism from many designers, which I think in the long run ends up producing a better amplifier.
2.)A lot of the designs that are out there if you scour the web tend to be less than user friendly, particularly with regard to using rare parts, using transformers that aren’t intended to be output transformers, or using custom wound transformers. Although the custom made route is probably the best in this case, I wanted to use something “off the shelf”, simply because I feel more people are likely to buy and experiment with an available product than to go through the hassle of getting something custom made, even if the price difference isn’t all that large.
3.)It’s been my (unconfirmed) guess that many of the commercial SET designs out there use a resistor in parallel with the load on the secondary of the output transformer. This “trick” allows the amp to drive headphones of multiple impedances without having to use different secondary taps. The headphones are in parallel with a low-ohm resistor (typically 4-8R). Because most headphones are much higher impedance than the resistor, the overall impedance of the load tends to be about that of the resistor alone, which means that headphones of any impedance can be used, as long as their impedance is typical (30-600R). The obvious drawback here is that a huge amount of power is wasted across the resistor. SET amplifiers are already notoriously inefficient as it is, and I simply find this resistor method to be an inelegant engineering solution. As such I wanted to use a transformer with secondary taps such that they could be matched to a specific headphone. Designing some kind of tapping scheme such that the amplifier could be used with more than one type of headphone is even better.
All of the above led me to attempt to put together something that is at least a start on a DIY SET transformer-coupled headphone amp. NOTE: I AM NOT AN EE. I am an enthusiast who has read a few books and is trying to piece something together that will sound good and operate safely and with good stability. As such I don’t expect this in its current form to be prefect, or anywhere close, to be honest. I am doing this as much as a learning experience for me as for anything else. I welcome ideas, criticism, etc., as long as they are constructive.
Project: A DIY SET Transformer-Coupled 2A3 Tube Amplifier
Goal: The design of a high quality transformer coupled headphone amp using current-production or easily obtainable parts, including the output and mains transformers. Because the iron will be expensive, no effort will be made to go cheap on the other components or the design, however, complexity at a high cost to performance ratio will be avoided. The amplifier should be easily configurable to drive “common” headphones…. 32R, 300R, etc. Being a SET amplifier, there won’t be a huge emphasis on low distortion, etc. However, there will be an emphasis on keeping distortion of the even-order variety, and where it is easy to minimize, steps will be taken to do so.
The first and most limiting choice is the output transformer. Because I wanted to use an off the shelf model, this limits the design choices in the output stage. Also, there simply aren’t that many OTs available for headphones. Many other designs I have seen have chosen to use interstage transformers, as there are more of them on the market. I have chosen to use one of Sowter’s headphone OTs. I decided this because they are designed for SET headphone amps, they have taps for many common headphone impedances, and because I haven’t seen them used in other designs I have come across, and it seems a shame to have a transformer on the shelf and ready to go and not used in common DIY designs.
For the output stage I chose to use a 2A3. This was based on a suggestion from another forum member. I’m not an experienced tube roller and didn’t really have a preference on the output tube; however, I’ve read great things on the 2A3 and as I said it was suggested to me by another member who is much more experienced than I am at this. There are current production models, and you can get a decent sounding matched set of Sovtek’s for 70-80$.
I designed the output stage around the suggested operating point of Va = 250V, Iq = 60mA, and Vgk = -45V. This choice required the Sowter 8976. The 8976 has a primary Z of 2k, with secondary Z configurable to 300, 150, 75, and 18 ohms. This is achieved with 4 equal secondary windings. Applying them in series gives a 2.5:1 turns ratio, while applying them in parallel gives a 10:1 turns ratio. Different combinations of series and parallel application allow matching the headphone impedance to the desired primary impedance. The output stage requires this transformer because it is the only off the shelf Sowter headphone transformer than can handle the quiescent anode current of 60mA.
I’m intending this amplifier to drive my 32R Grado HF-2s. As such with a 10:1 turns ratio the transformer presents a 3200R impedance to the output stage. Sennheisers can achieve nearly the same impedance by wiring two of the secondaries in parallel and then in series with the rest of the secondaries, giving a 3.3:1 turns ratio and a load impedance of 3333R. The plate characteristics of the output stage are presented below, with the loadline, maximum anode power, operating point, and maximum achievable voltage swing all marked (for 32R headphones).
I’ve chosen to use a resistor in the cathode circuit to bias the output stage and bypass it with a capacitor. I’m open to suggestions on other arrangements here, however I’m actually pretty comfortable with this method of biasing. The negative feedback at DC keeps the circuit stable and at intended operating parameters better than other biasing methods, and I don’t desire the use of a negative power rail to bias the grid directly. The required capacitor for a f-3dB is 42uF. The total voltage swing available is 273 Vpk-pk, which translates to a full ouput power of 2.9W into 32 R headphones. The predicted distortion of the output stage is approximately 5.5%, composed mostly of second order harmonics. This distortion falls linearly with output power, so at 1/10th the output power, distortion would be 0.5%, about typical of a SET amplifier. Gain of the stage is approximately 3 (before being reflected through the transformer).
The chosen operating point of the output stage is with a Vgk = -45. For maximum output swing, the stage must be driven by 32VRMS. The input stage must therefore have an input sensitivity of approximately 2VRMS (to be driven by the line-out of most DACs on the market today), a gain of at least 16, and an output impedance of less than 11k (this was calculated based on the Miller capacitance of the output stage and a f-3db of 150kHz).
I’m fairly pleased with the output stage. The driver stage is another matter. There are many choices available to use in this stage. For this first iteration, I’ve decided to start off simple. We can enhance the stage to make it better from here. For now, the design uses a 6DJ8, and is nearly stolen from Regal’s mod to runeight’s Bijou. I’ve modified the chosen operating point slightly to Va = 108 and Iq= 5mA. This requires a RL of 28k4 and a HT of +250 V and gives a Vgk of -3V. This was chosen based on the output voltage of my DAC (1.7VRMS). This voltage gives a maximum Vpk-pk of 4.8. I wasn’t able to find in any 6DJ8 datasheets at what Vgk the onset of grid current begins but I believe it is around -1V for most tubes. Under most operating conditions the input signal should remain at or below 4 Vpk-pk, so Vgk of -3 seemed to be a reasonable choice. At this operating point the calculated rp was 5k2. As is, the current stage has a gain of 28, approximately the correct input sensitivity, and an output impedance of 4.4k, all of which fall into our needs for driving the ouput stage.
Improvements should be made to the input stage before moving on with the design. Currently, the gain is really too high to drive the output stage, and the stage is noisy and will cause a large amount of distortion (comparatively). I think that this can all be improved with the use of some negative feedback and/or an active load for the stage. I’ve briefly considered a u-follower, B-follower, or SRPP stage in place of the current design, but haven’t really tackled those till I’m sure that the added complexity is really needed. Any suggestions would be greatly appreciated.
Here is a schematic for the amplifier in its current form. Note: I have sent a request to Sowter to inquire about the DC resistance of the primary of the 8976. This is required before the final value of the HT can be known. After I get this info I’ll post my thoughts on a power supply and its options. Also note I mislabeled the cathode voltage of the output stage. This should be 45V, not 50.
I don't have any of the software to simulate this circuit. I would appreciate anyone that does letting me know what it looks like. I'd also appreciate lots of input, particularly on improving the input stage, but any comments will be helpful.