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PostPosted: 08 Oct 2010, 17:08 
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Benny

Forget the hum test, nice old vintage phones. Classics. I have never really chased hum to that level. More if hum is high when near the speakers I will chase it down in the amp until it is very low. Tube amps and hum appear to go together to some degree, especially SE amps.

The only tube amp I have built that had no audible hum at all was the Be Bamp. Dead quiet. My current SET has a tiny bit which can only be heard with your ear against the speaker. At the 1 meter test poit, zero!

My son's KT88 Merlot had a lot of hum until I re-ran the earth wires to the input RCAs. I was using the top plate to carry earth to them originally. But until I ran a wire direct from the PS negative to the RCA ground the hum was too loud. All good now.

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PostPosted: 09 Oct 2010, 07:58 
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mwhouston wrote:
What sensitivity have Your speakers

Forget the hum test, nice old vintage phones. Classics. I have never really chased hum to that level. More if hum is high when near the speakers I will chase it down in the amp until it is very low. Tube amps and hum appear to go together to some degree, especially SE amps.

The only tube amp I have built that had no audible hum at all was the Be Bamp. Dead quiet. My current SET has a tiny bit which can only be heard with your ear against the speaker. At the 1 meter test poit, zero!

My son's KT88 Merlot had a lot of hum until I re-ran the earth wires to the input RCAs. I was using the top plate to carry earth to them originally. But until I ran a wire direct from the PS negative to the RCA ground the hum was too loud. All good now.

What sensitivity have Your speakers? My speakers have 96 Db and they are very good test for my amplifiers /it's very hard to lie it!/, and I do everything to make hummmm-free ampls. I know that with another regular speakers people doesn't sense hummmm!

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Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
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PostPosted: 11 Oct 2010, 11:23 
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Hi everybody,

I chase hum with high sensitive headphones and load resistors. Those are crystal set headphones with 4000 ohm impedance, there is nothing more sensitive than that. If I claim something noise free, it is noise free, those phones hear things my O-scope won't see, so to me this is the way to go. With those headphones you can listen to your local am radio station with 20 yards of wire and a germanium diode, nothing else, I am serious. Having high impedance headphones gives you entire new possibilities in tracing signal and noise, direct connected to the record player you hear the music loud and clear, without RIAA equalization, of course, but still, this is sensitive, I suppose. I build my amps with Oscilloscope and Analyzer, but when I am all done, I take out those headphones and see how noise free it really is. If you have no oscilloscope, such a set of headphones will do just as good or better, best option IMHO is to use both.
Attachment:
Vintage Headphones (450 x 600).jpg

All the best.


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Greetings from Holland, Ben

My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 12 Oct 2010, 21:30 
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My apologies to those who have been waiting for my next design installment on this thread. I have been traveling and then I returned sick :sick: and have been recovering. In a couple of days I should have the discussion on setting the overall frequency response of the amp and the final schematic. Following that, the PS schematic should come shortly. Thank you all for your patience. :up:

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PostPosted: 12 Oct 2010, 21:37 
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Hi Matt,

get well mate!


All the best.

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Greetings from Holland, Ben

My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 13 Oct 2010, 04:51 
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Suncalc wrote:
My apologies to those who have been waiting for my next design installment on this thread. I have been traveling and then I returned sick :sick: and have been recovering. In a couple of days I should have the discussion on setting the overall frequency response of the amp and the final schematic. Following that, the PS schematic should come shortly. Thank you all for your patience. :up:

The world awaites!

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Projects:”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp | "LeManja” - hybrid tube and chip headphone amp


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PostPosted: 14 Oct 2010, 16:40 
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Cool thread. All I can say is the 300B makes a great sounding SE amp. I love mine, and it uses 6SN7 drivers too. I didn't spend a lot of time tweaking the design, just used one of the circuits already out there that's self biasing. This is an excellent power amp to build for beginners (yes, I said beginners) IMHO. The theory of operation is very basic, as are the simple common cathode triode stages. There's not a lot to go wrong and it's easy to troubleshoot.

I AM very curious to hear a review of the difference in sound between the expensive carbon plates and the EH version. I use EH 300Bs and they sound great in my setup, but then again I haven't compared them to any other 300Bs.

I use 30 watt Hammond OTs, figuring the extra iron would provide a better, less distorted low end than a lower wattage tranny. BTW, Edcor has yet to publish their maximum allowable bias currents for their SE OTs. The word I got from them was they were planning to do so soon, but apparently not, at least not last time I checked, although Suncalc already licked that problem in another thread with some clever calculations. Go Matt!

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PostPosted: 14 Oct 2010, 18:54 
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I got some EH 300Bs also for testing and comparison (plus a spare).

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PostPosted: 16 Oct 2010, 00:41 
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I seem to have recovered my health and as such have tackled the issue of setting our overall frequency response for our 6SN7/300B amplifier. However, before I begin with the details, I believe that a few comments on frequency response in general are appropriate.

The required frequency response of an amplifier is a careful trade off between the necessary bandwidth to reproduce the signal to be amplified and the signal distortion and degradation which results from an excessive bandpass characteristic. If the bandwidth is too low (or misplaced), critical content will be stripped from the signal. If the bandwidth is too large, the amplifier can suffer from bias excursions, intermodulation products, and high frequency oscillation.

Setting the frequency response also requires a knowledge of the signal to be amplified. In audio, the required bandwidth is often assumed to be 20Hz to 20KHz. However, even this bandwidth is often excessive in real world situations. The lowest note produced by an electric Bass is 43Hz, most percussion doesn’t produce anything below 30Hz, and even the lowest key on the piano (A0 Double Pedal A) has a minimum fundamental frequency of 27.5 Hz. Some synthesizers produce lower tones, but rarely. In addition, the limits of human hearing leave something to be desired at very low and very high frequencies. The figure below shows the sensitivity of human hearing across the frequency spectrum for the 5th, 50th, and 95th percentile population. (Reproduced from “Hearing, the Determining Factor for High-Fidelity Transmission”, Harvey Fletcher, Proceedings of the I.R.E, June 1942)
Attachment:
Hearing contours.jpg

Using the reference of 1000Hz, a 95th percentile person (i.e. someone with truly excellent hearing) still has a sensitivity rolloff of over 70dB at 30 Hz and over 100dB at 20Hz. Also of note is the upper hearing tolerance for most people. The average person has an upper frequency hearing limit in the neighborhood of 16kHz to 18kHz. It is for these reasons that the historical definition for high fidelity (prior to the “specification wars” of the 1970s) was typically a bandpass with the half power points at 30Hz and 18kHz. During the 1970s, as marketing people attempted to market their new transistor amplifiers, the “definition” for high fidelity soon became the more easily marketed “20 to 20k” frequency response (Also more easily remembered by stereo salesmen). And somewhere in the midst of all the marketing, the definition also went from a half power bandwidth (i.e. the -3dB single pole roll off points were at 20Hz and 20kHz) to “flat from 20 to 20k”. All with preposterous THD numbers which were orders of magnitude below the level of human perception.

I bet you’re wondering where I’m going with all of this. Well, I need to decide on the bandwidth requirements for this amp and I thought a little hard data and historical perspective would help things along. So, my decision was to attempt to achieve the 20Hz to 20kHz -3dB bandwidth (but not the “flat from 20 to 20k” bandwidth). This is a good balance between what is actually required in the real world and what people have come to “expect”. My reasons for this decision should become apparent shortly. (As a side note: I seriously recommend that anyone building amplifiers have a licensed audiologist give them a hearing test and produce an audiogram for both ears. It is amazing what you may find out. You don’t have to show it to anyone but it is very good to have an honest unbiased assessment of what you can actually hear.)

Up to now I have discussed the amplifier stages and components values but have not provided any kind of a schematic. This is intentional. I wanted to focus on the design process and sometimes the immediate generation of a schematic detracts from the discussion. However, I have arrived at a point where it is now necessary to begin looking at the amplification chain as a whole and this requires a schematic. The following figure is a basic hand drawn schematic of a single channel with pertinent values written in.
Attachment:
300B_schematic_preview.jpg

You will note that (among other careful omissions) the values of the two cathode bypass capacitors and the coupling capacitor are missing. These three capacitors set the lower limit of the amplifier frequency response for normal operation.

Often times one will see an amplifier designer simply choose the largest values possible for these capacitors in order to maximize bandwidth. The pitfall in this approach is that this assumes that the amplifier will always be operating in a controlled fashion and within its design limits. Unfortunately, the music being played through the amplifier doesn’t really care about the design limits of the amplifier, it is what it is. Some music may have volume spikes that rise 10dB or 20dB above the average level of the music. If the amp is being driven hard, these spikes will exceed the input limits of the amplifier. When this happens the values of these capacitors will have a profound effect on what happens to the transient response and the distortion produced.

The key to setting the bypass and coupling capacitor values lies in understanding what happens when the amplifier is momentarily pushed beyond its design limits. Looking back at the driver stage, if the instantaneous driver stage output voltage exceeds the output stage bias voltage, the grid of the 300B will start to draw current. (i.e. the input impedance of the output stage goes from a very high impedance to a very low impedance.) When this happens the DC voltage across the coupling capacitor will be pushed up by the added current flowing through it. The cathode bypass capacitor will likewise drift because of the same added current and it’s DC voltage will rise as well. Then when the input voltage falls below the added dc component on the capacitors, the input to the tube will no longer follow the output of the driver stage and the signal will be lost. This condition will persist until the added charge on the capacitor is drained off (through the grid resistance of the output stage). This phenomenon is called bias excursion and bias recovery and it occurs whenever the output stage is overdriven. (For those interested in the details of this phenomenon, there is an excellent explanation in Chapter 8 of “Guitar Amplifier Power Amps”, by Richard Kuehnel. I highly recommend this text for anyone building power amps be they for guitar or for high fidelity.)

The key thing to remember here is that the time it takes for the amplifier to fully recover from a bias excursion is approximately five times the RC time constant of the coupling capacitor and equivalent grid circuit resistances. (As well as the time constant of the bypass capacitor and cathode impedance.) This means that if the capacitor values are too large, when the amp is overdriven it will noticeably blank for short periods of time. Like everything else, this is a tradeoff.

I like to use what I call the 400 beats per minute test. (400 bpm is about the fastest tempo you’ll commonly find in most music.) If a bias excursion occurs, I don’t want to “miss a beat” as it were. This means that I want the bias excursion recovery time to not exceed one beat at this fast tempo. This places the five time constant limit at 150mS. Below is a spreadsheet snapshot which I like to use to calculate the overall response for two stage amps. It shows the combined rolloff vs frequency as well at the bias excursion recovery time for each stage.
Attachment:
Rolloff spreadsheet.jpg

I have inserted the equivalent cathode impedances in the stage 1 and stage 3 sections and added the total equivalent coupling circuit resistance in the stage 2 section. Using this sheet I select cathode capacitors which give the bias excursion recovery time I want and it tells me the total combined rolloff as a function of frequency. I then juggle the three capacitor values to get the overall performance I desire. You may notice that the input stage capacitor was made larger to get back to the desired half power response at 20Hz. This was done at the expense of bias excursion recovery time in the input stage. The is all right however because this amp is designed such that the output stage will go into severe breakup well before driver stage. As such we really don’t need to be that concerned about the driver stage bias excursion response. The resultant capacitor values are Ck1=100µf, Ck2=50µf, and CC=0.1uF. In spite of these values being significantly smaller then sometimes seen in amp schematics, a quick review of the spreadsheet will show that the relative 20Hz response is -5.93dBv or -2.97dBW exactly as desired.

The only item left is to set the high frequency response of the amplifier. Often time designers will allow the upper frequency response to run into the hundreds of kilohertz. Where as this might look impressive on paper, the truth is that this type of response means that your amp will be amplifying high frequency noise and may even be oscillating at high frequency. At best it is a source of additional intermodulation distortion and at worst it may be robbing you of output power otherwise useful in the audio band. It is a much better practice to limit this response to a reasonable audio frequency.

The input capacitance of our output stage at this bias point is approximately 82pf (or 82µµf for you traditionalists out there). The high frequency response is set by this capacitance driven by the high frequency transfer impedance between the stages. This resistance is the driver stage rp in parallel with the driver stage RL in parallel with RG of the output stage. In our case this is 11.4kΩ||50kΩ||250kΩ=8.95KΩ. This gives us a half power point of 216kHz. We want to bring this value down to something more reasonable. This is accomplished by adding a grid stopper resistor on the 300B. This resistance simply adds to the transfer impedance directly in series. By adding a 20kΩ grid stopper resistor, the high frequency half power point is reduced to slightly over 67kHz (a much more reasonable value). The response is about -1.1dBW at 20kHz. Because we don’t know the output impedance of the preamp it is impossible to determine the high frequency rolloff on the driver input. However, with the 6SN7 Miller capacitance being 69pf, the preamplifier output impedance necessary to place the half power point at 20kHz is approximately 115kΩ. So a quick calculation will show that so long as the preamp output impedance is less then 62kΩ (including the parallel 200kΩ input grid resistance) (fH=37.2kHz) then the total amplifier will achieve its 20kHz half power upper rolloff.

So now we have intelligently set the frequency response of our amp and done it using standard values for all the capacitors in the circuit. Below is the hand drawn schematic of a single channel of the amp.
Attachment:
frequency schematic.jpg

The only other change is the that the grid resistor on the 300B has been reduced to 220K to keep below the maximum specified grid resistance for this tube. The 12% reduction should not have an appreciable effect of the overall operation of the amplifier. A quick recheck of the various design calculations bears this out.

Next time (hopefully in just a few days) I should have the final power supply design completed and a final schematic for the entire amp. Once again, thank you everyone for your continued patience. Especially Mark who has been waiting very patiently for me to finish this design.


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PostPosted: 16 Oct 2010, 04:17 
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Matt, Glad to see you have not gone to the big listening lounge in the sky. Thanks for the work so far. Once I have power tranni specs (and hopefully you suggest an Edcor power tranni) and choke specs (Hammond) I can order the OPTs and power tranni from the States. AU Dollar is good for us at the moment so hopefully I can order soon.

Thanks once more.

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Projects:”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp | "LeManja” - hybrid tube and chip headphone amp


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