I do not use C13. Use a Cinemag transformer here. For normal gain a CM-15/15B line input transformer. Mount it where C13 is supposed to go. Been using a 20K resistor for R13 to load properly.
To better match the amplifier gain of my three way horns I use a Cinemag CM 10K/600 line input transformer. This is a 4:1 step down, has -12 dB attenuation. Use a 2.4 K resistor to load it at R13.
I need to post some RTA of a cap, 1:1,and 4:1 step down. I like using an input transformer instead of a capacitor. It also does the balanced to single ended conversion. My sources are all balanced output.
To better match the amplifier gain of my three way horns I use a Cinemag CM 10K/600 line input transformer. This is a 4:1 step down, has -12 dB attenuation. Use a 2.4 K resistor to load it at R13.
I need to post some RTA of a cap, 1:1,and 4:1 step down. I like using an input transformer instead of a capacitor. It also does the balanced to single ended conversion. My sources are all balanced output.
I've done that on a valve project. It is a viable alternative and sounds nice, although it does give up some bandwidth. I use a simple op-amp circuit as a balanced line receiver on FE.
Its interesting that controlling DC seems to be a key issue in amp design. Mauro used a blocking capacitor, C13, plus the C9 gain roll off in My_Ref and a DC servo in the EVO. Neither are a perfect solutions. Some FE and EVO guys are willing to take the risk and run DC coupled (no cap or servo). Schiit has gone to microprocessor monitoring of DC offset, transistor bias, temperature, and faults. This claims to eliminate the need for caps or dc servos.
Anyway, an interesting topic. Thanks for sharing your setup with input transformers.
Jac
I would like to ask for some explanations about the input transformer. I admit my lack of technical knowledge.
From what I understand, a transformer blocks the passage of DC (the generic concept that a transformer separates / isolates is not new to me).
In my stereo system, all the sources (dac, tuner, pre-phono) are connected to a passive TVC transformer from Glasshouse and this is connected to My_Ref Evo A.
In this case, passing the source signal through the passive pre transformers, is DC blocked? Could I safely remove C13?
If not, why? I would like to understand and learn ...
Thank you for your patience (the novices are always annoying! ...)
Ciao
Giacinto
From what I understand, a transformer blocks the passage of DC (the generic concept that a transformer separates / isolates is not new to me).
In my stereo system, all the sources (dac, tuner, pre-phono) are connected to a passive TVC transformer from Glasshouse and this is connected to My_Ref Evo A.
In this case, passing the source signal through the passive pre transformers, is DC blocked? Could I safely remove C13?
If not, why? I would like to understand and learn ...
Thank you for your patience (the novices are always annoying! ...)
Ciao
Giacinto
I agree with George, give it a try.
I am no expert, but I was taught that transformers in audio are sometimes difficult creatures. When they are unhappy, they can make this worse. In particular, signal transformers are fussy about the impedance that follows them. For example, here is the datasheet for the transformer that Panelhead mentioned that he uses. It's the same that I used in a valve phono pre-amp project.
http://cinemag.biz/line_input/PDF/CMLI-15-15B.pdf
As he said, he replaced R13 (100k) with a 20k resistor to correctly load the transformer and doesn't use C13. Please notice the graphs in the datasheet showing bandwidth and noise.
A capacitor in series with a resistor as a load for the transformer can do funny things. Often no capacitor or a small (pF) capacitor can give the best response, but larger capacitors can be a problem.
Your experiment, per George's suggestion, should be very interesting. Also, if you have measurement capability, you might find some interesting results.
Jac
I am no expert, but I was taught that transformers in audio are sometimes difficult creatures. When they are unhappy, they can make this worse. In particular, signal transformers are fussy about the impedance that follows them. For example, here is the datasheet for the transformer that Panelhead mentioned that he uses. It's the same that I used in a valve phono pre-amp project.
http://cinemag.biz/line_input/PDF/CMLI-15-15B.pdf
As he said, he replaced R13 (100k) with a 20k resistor to correctly load the transformer and doesn't use C13. Please notice the graphs in the datasheet showing bandwidth and noise.
A capacitor in series with a resistor as a load for the transformer can do funny things. Often no capacitor or a small (pF) capacitor can give the best response, but larger capacitors can be a problem.
Your experiment, per George's suggestion, should be very interesting. Also, if you have measurement capability, you might find some interesting results.
Jac
Hi Giacinto,
Silmic II (The black/gold ones, though) is a valid choice for C9 for whom prefer musicality vs detail.
But sincerely Silmics' loss of detail is really too much...
I quote Jac, a BlackGate PK could be a really nice alternative giving you both detail and musicality.
BTW in general I would do a sanity check in the rest of your setup, usually harshness and lack of musicality are elsewhere...
Hi Dario, your suggestions are always welcome.
I'm sure there is no harshness in this amp with the components shown in the bom and tutorial. On the contrary, all the musical qualities are there.
But in my practice and custom of listening to music, playing all genres and also many recordings that are not technically valid, but whose contents are important to me, I consciously need to give up a little detail in favor of an overall musicality.
In other words, harshness is in some software I play, not in the amp or in the rest of the system. If I can have detail and don't suffer harshness from software, welcome.
In this regard, the first plays without C13 (given my use of a passive preamp TVC and your advice I decided to try) are very interesting. I need more days of listening and playing many different recordings, but first impressions are of a natural and coherent sound, which reveals more detail and is at the same time sweeter. It would seem to be in the direction of what I was looking for.
I wouldn't want to exaggerate after a few hours of listening, but I also have the impression that something related to the phase is much improved. Everything seems to be going to the right place, even with the worst software I'm not bothering. Indeed, I would say that right now, listening to music, I am feeling very pleased, sto godendo! If these impressions are confirmed, I will try again a more detailed cap in C9.
Unfortunately, in addition to a tester, I have no tools to make measurements that can support my impressions. I will update you.
Thank you all
Giacinto
I'm sure there is no harshness in this amp with the components shown in the bom and tutorial. On the contrary, all the musical qualities are there.
But in my practice and custom of listening to music, playing all genres and also many recordings that are not technically valid, but whose contents are important to me, I consciously need to give up a little detail in favor of an overall musicality.
In other words, harshness is in some software I play, not in the amp or in the rest of the system. If I can have detail and don't suffer harshness from software, welcome.
In this regard, the first plays without C13 (given my use of a passive preamp TVC and your advice I decided to try) are very interesting. I need more days of listening and playing many different recordings, but first impressions are of a natural and coherent sound, which reveals more detail and is at the same time sweeter. It would seem to be in the direction of what I was looking for.
I wouldn't want to exaggerate after a few hours of listening, but I also have the impression that something related to the phase is much improved. Everything seems to be going to the right place, even with the worst software I'm not bothering. Indeed, I would say that right now, listening to music, I am feeling very pleased, sto godendo! If these impressions are confirmed, I will try again a more detailed cap in C9.
Unfortunately, in addition to a tester, I have no tools to make measurements that can support my impressions. I will update you.
Thank you all
Giacinto
Giacinto,
It took me a while to remember that TVC stands for transformer volume control. From there, I went to glasshouse to look at their TVC transformers. Very nice, but limited data, so I went to Sowter who is famous for their high quality transformers. They say that their TVC (they call them transformer attenuators) can drive any load impedance, but the higher the better.
SOWTER TRANSFORMER ATTENUATORS TVC
They also say that, "Where necessary a blocking capacitor should be used to ensure no dc is applied to either winding." And, "The transformers provide good dc isolation and can be used in balanced or unbalanced circuits." I will let you decide what they mean.
It still seems like a good idea to try shorting C13 and see how it sounds. If you like it, you could try 20k at R13, but it doesn't sound so necessary based on Sowter's information.
Jac
It took me a while to remember that TVC stands for transformer volume control. From there, I went to glasshouse to look at their TVC transformers. Very nice, but limited data, so I went to Sowter who is famous for their high quality transformers. They say that their TVC (they call them transformer attenuators) can drive any load impedance, but the higher the better.
SOWTER TRANSFORMER ATTENUATORS TVC
They also say that, "Where necessary a blocking capacitor should be used to ensure no dc is applied to either winding." And, "The transformers provide good dc isolation and can be used in balanced or unbalanced circuits." I will let you decide what they mean.
It still seems like a good idea to try shorting C13 and see how it sounds. If you like it, you could try 20k at R13, but it doesn't sound so necessary based on Sowter's information.
Jac
You're welcome
This is an error I've also did in the past.
Colouring the sound to 'fix' recordings errors works just for the single record and will sound wrong/bad for all the others.
Only after some time I've learnt that records (good and bad ones) best friend is transparency.
It's for this that I've revised completely the BOM some years ago.
You see...removing C13 you removed a great source of colouration and things are better now.
C13 in general is still needed but this test could make my point, isn't it?
Dario, yes it is.
Coloration is the key word, I think. Without C13 I have more detail and transparency, but without the coloration that make to my ears a sensation of harshness (but maybe these are not the exact words to explain) with some bad recordings. For this reason I said I will try again more transparent sounding caps in C9 as your recommendations for cerafine, kz, bg etc.
Now my problem is to know if I can play without C13 in complete safety, without the danger that DC passes, relying on the isolation of the transformers of my passive preamp.
I also read on the Sowter website (thanks to Jac for studying and for information) and indeed there is an ambiguity.
For this reason some posts back I asked for information and explanations about the ability to block DC of transformers like those used in my preamp.
For now everything works fine after more than 10 hours without C13 and I'm still listening with great pleasure. So I think the encouragement to try by Joseph K is just right and I thank him again.
But a technical explanation to understand will always be appreciated.
Ciao a tutti
Giacinto
Coloration is the key word, I think. Without C13 I have more detail and transparency, but without the coloration that make to my ears a sensation of harshness (but maybe these are not the exact words to explain) with some bad recordings. For this reason I said I will try again more transparent sounding caps in C9 as your recommendations for cerafine, kz, bg etc.
Now my problem is to know if I can play without C13 in complete safety, without the danger that DC passes, relying on the isolation of the transformers of my passive preamp.
I also read on the Sowter website (thanks to Jac for studying and for information) and indeed there is an ambiguity.
For this reason some posts back I asked for information and explanations about the ability to block DC of transformers like those used in my preamp.
For now everything works fine after more than 10 hours without C13 and I'm still listening with great pleasure. So I think the encouragement to try by Joseph K is just right and I thank him again.
But a technical explanation to understand will always be appreciated.
Ciao a tutti
Giacinto
OK, I really am not an expert on this, but I will try a possible explanation. I hope that others with more knowledge will chip in and improve our knowledge.
My understanding is that, for the small amounts of DC in the normal signal path, the TVC does a great job of isolating DC and can effectively replace C13. Sowter's comment about possibly needing a DC blocking cap to protect the transformer windings probably refers to what happens to a transformer, especially a toroidal transformer, when a higher level of DC is introduced.
When DC is fed into a transformer, there is no inductive reactance because the DC is a constant current. That means that the low resistance of the winding are the only thing resisting the current. The result is high current flow that can damage the winding if the DC voltage is high enough.
If we are feeding the transformer with DC + music (AC signal), the DC causes an offset that can saturate the core for a partial cycle. That can result in noise from the transformer itself. Actually, the worst condition is when the system is on, but there is no AC input. Under this condition, even a small amount of DC will cause core saturation.
The sensitivity to saturation is a function of transformer design. EI transformers can be less sensitive, depending on the gap between the core elements, the E and the I. Toroids, among others, have continuous cores that can't leak off the saturation flux and are easier to saturate.
In sum, the safest way is to keep a DC blocking capacitor, plus your TVC. That said, assuming low DC from your source, using the TVC without a DC blocking cap should provide good DC isolation for normal conditions. Its only under a failure that you would risk damaging components.
Ok you guys with more knowledge than I. Please correct my mistakes and add better explanations.
Jac
My understanding is that, for the small amounts of DC in the normal signal path, the TVC does a great job of isolating DC and can effectively replace C13. Sowter's comment about possibly needing a DC blocking cap to protect the transformer windings probably refers to what happens to a transformer, especially a toroidal transformer, when a higher level of DC is introduced.
When DC is fed into a transformer, there is no inductive reactance because the DC is a constant current. That means that the low resistance of the winding are the only thing resisting the current. The result is high current flow that can damage the winding if the DC voltage is high enough.
If we are feeding the transformer with DC + music (AC signal), the DC causes an offset that can saturate the core for a partial cycle. That can result in noise from the transformer itself. Actually, the worst condition is when the system is on, but there is no AC input. Under this condition, even a small amount of DC will cause core saturation.
The sensitivity to saturation is a function of transformer design. EI transformers can be less sensitive, depending on the gap between the core elements, the E and the I. Toroids, among others, have continuous cores that can't leak off the saturation flux and are easier to saturate.
In sum, the safest way is to keep a DC blocking capacitor, plus your TVC. That said, assuming low DC from your source, using the TVC without a DC blocking cap should provide good DC isolation for normal conditions. Its only under a failure that you would risk damaging components.
Ok you guys with more knowledge than I. Please correct my mistakes and add better explanations.
Jac
When I previously used a TVC preamp, I learned that my Sonic Euphoria preamp employed autoformers, not transformers, to control the output voltage. An autoformer has only a single coil and does NOT isolate DC input from the output.
Be careful if your passive preamp uses an autoformer for its volume control. Most TVC's are actually AVC's, and the term is used incorrectly.
If you can't measure DC, you could try using the shorting wire as suggested, but attach only one end to one of the capacitor leads. With the amp powered, but no music playing, touch the other end of the shorting wire to the other capacitor lead. If you hear a click or pop or see the woofer cone jump, you have DC coming from your preamp. It will not damage the woofer, but it might make you jump! If your speakers have a passive crossover, the caps in it will protect the tweeter.
Peace,
Tom E
Be careful if your passive preamp uses an autoformer for its volume control. Most TVC's are actually AVC's, and the term is used incorrectly.
If you can't measure DC, you could try using the shorting wire as suggested, but attach only one end to one of the capacitor leads. With the amp powered, but no music playing, touch the other end of the shorting wire to the other capacitor lead. If you hear a click or pop or see the woofer cone jump, you have DC coming from your preamp. It will not damage the woofer, but it might make you jump! If your speakers have a passive crossover, the caps in it will protect the tweeter.
Peace,
Tom E
Thanks for explanations. Glasshouse TVC passive preamp employs transformers, not autoformers. These transformers are not toroidal but EI. I suppose they were similar to Promitheus Audio trafos.
Do you can explain how to measure DC? Can I use a simple voltmeter to read the signal output from preamp?
If for security reasons I had to replace C13, in order to reduce colorations, could I use a value less than 1uF?
I have some CDE 940c 3000vdc capacitors 0.01uF and I could try them. In the range of nanofarads I can find better quality and technology capacitors within the appropriate budget and size, I think.
Ciao
Giacinto
Do you can explain how to measure DC? Can I use a simple voltmeter to read the signal output from preamp?
If for security reasons I had to replace C13, in order to reduce colorations, could I use a value less than 1uF?
I have some CDE 940c 3000vdc capacitors 0.01uF and I could try them. In the range of nanofarads I can find better quality and technology capacitors within the appropriate budget and size, I think.
Ciao
Giacinto
That's right. It should be very small because the amplifier increases it by 30 to 1. As I understand your TVC, you won't find any DC at the output of the TVC because the transformer won't pass DC.
A smaller value at C13 will degrade the bass. C13 and R13 form a high pass filter that roll off the bass at about 1.6 Hz (f= 1/2piRC). Mauro chose this to be at a low frequency so that the bass doesn't see much phase change due to the filter.
If you were to use a 0.01 uF at C13, the bass would roll off at about 160 Hz.
Jac
I made a measure, I hope with the right method.
Dac turned on without a music signal connected to the preamplifier. Half-way volume. Voltmeter connected, as shown in the attached image, to the connection cables for the power amplifier (obviously not connected to the power amp).
Measured on both channels 0.00 millivolts, as expected.
If I have not the wrong method, I am happy with this confirmation and I will enjoy the My_Ref without C13.
Ciao a tutti
Giacinto
Dac turned on without a music signal connected to the preamplifier. Half-way volume. Voltmeter connected, as shown in the attached image, to the connection cables for the power amplifier (obviously not connected to the power amp).
Measured on both channels 0.00 millivolts, as expected.
If I have not the wrong method, I am happy with this confirmation and I will enjoy the My_Ref without C13.
Ciao a tutti
Giacinto
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