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What tube to use, and on which voltage? Based upon the experience of others the ECC88/6922 seems to be a good choice since it is stable at "lower" plate voltages. Too bad, I have some ECC83 but these have too much gain for this application, ECC82 maybe .. or a 6N1P same as used in my UL40s2 amp (same pin-out as the 6922).
I'll go for the ECC88/6922/6N1P tube and order a pair of 6922 from JJ from Automatic Electric. As for the amp itself, I have a few LM3886 around which I did not use until now, so this would be a perfect application for these chips. I will use the LM3886 op amp in inverted mode, and in order to be not dependent on tube aging I will use higher values for the feedback resistor and the gain resistor.
Below you will find a few of the options I've considered. Please do not build them without proper testing, since I've calculated and simulated all of them, and they'd probably work. However I will only build one of them and building one of the others therefore is not a follow-the-numbers project.
The most simple version of a tube buffer at the input instead of an opamp buffer or just a bare attenuator is one making use of a basic Cathode Follower circuit. After all, we do not need any gain, just a buffer with stable and reasonably low output impedance. This would be a possible circuit. Others, like Joe Rasmussen use similar designs, but apart from values of cathode resistors, there are not many places where you can go wrong.
Resistor R2 must be chosen such that the Voltage between Cathode and Grid is acceptable for the 6922 tube. Something in the range of 10-22 kOhms is OK.
But, as mentioned on the previous page, I would like to substitute the standard cathode resistor with the other halve of the tube which will make the whole thing symmetrical wrt ground. The schema for the amp is as follows (for the moment that is):
I'm not entirely sure whether I need R2 giving grid of R2 a negative reference. Simulations show I probably don't need it, so watch this space.
The gain is set by R6/R4=470/22. And apart from the gain setting, which might be just a little bit higher due to the little loss in gain we have from the CF, there is only one thing to worry about: DC on the output. First of all, DC is blocked on the input of the power stage by audio capacitor C1. However, as explained in op amp background pages there is a need to level the impedance on both inputs of the op amp in order to avoid DC on the output as well. And since the inverted input does not have a direct connection to ground (absence of shunt resistor) R6 determines the impedance it sees. Therefore, R5 has the same value as R6 so that the DC offset on the output is minimized. The capacitor C2 on the non-inverted input will allow AC (=signal) to bypass freely.
Should I want to add a low-pass filter, then R4 might be substituted for two separate resistors with a cap to ground in between. Also possible to add a small cap parallel to the feedback resistor R6 although this changes the feedback again so probably not the best solution.
Well, according to Nelson Pass, adding current sources does make a difference in sound. A current source for the Cathode Resistor could be made with a bipolar transistor or a Jfet.
In the picture above the option with JFET is drawn, making the same design work with a bipolar resistor requires two additional diodes for biasing the transistor.
For those that need more output voltage there exists the possibility for a differential version. It's basically the same idea as the BP-100 bridged clone. It accepts differential input (only). I reduced the gain for both op amps so that the total gain remains within reasonable values for an amplifier.
For the moment I do not think that I'll use this version and go for version 2 instead.
I think I need a couple of transformers here:
In the next sections further detail is given for each of these power supplies.
Lets start with the power supply section for the LM3886 power opamps. As these little chips can consume lots of power, it requires a power regulator chip that's capable to provide a current of 3A or more. Negative voltage regulators are therefore out of the question so we'll have to work with two positive voltage regulators. Not too difficult as long as the power transformer has separate secondaries.
There are several regulator chips that come into mind:
The OPA549 is used in gainclone audio applications as well, but according to datasheets it's equally usable for building a regulated power supply.
Next question is of-course what voltage do we need for the amp. According to datasheet 35V would enable more than 60W with a load of 8 Ohms and still be able to handle loads of 4 Ohms.
Parts used may be any quality you like. Personally I plan to use the following components (per channel):
| Part no. | Description |
Price
|
| TR1 | Amplimo transformer type 58017, 160VA 2*30V |
€ 37.79
|
| SR1, SR2 | LM338 Voltage regulator 5A |
€ 3.50
|
| D1-D8 | Hexfret Diodes | |
| R1-R4 | 1 Ohms, 2Watt | |
| R5, R6 | 120 Ohms, 1/2 Watt | |
| P1, P2 | 5k pot, multi | |
| C1, C2 | Capacitor 6800uF/50V, or multiples of smaller values | |
| C3, C4 | Capacitor, 100uF/50V |
There are numerous good (and probably even better) ways to construct a regulated power supply. Please feel free/invited to improve my version for your own use.
If we aim at a B+ of around 40Vdc we need a transformer of 25-30Vac/1A depending on loss of Voltage regulator. Assuming the regulator works best with a Voltage drop of min. 2V we need 42V after rectification which means a power transformer with 28-30 Volts secondaries.
| Part no. | Description |
Price
|
| TR1 | Amplimo 28017, 50VA, 2*30V |
€ 27.54
|
Filament voltage for the 6922 is 6.3 Volts (AC or DC) +/- 0.3 Volts (The 6N1P +/- 0.6 Volts). The basic idea for power supply regulation is the same as described above for the amp. However, the circuit will be kept quite simple since the filaments are not directly in the signal path (as is the plate supply).
As said above, should this be necessary then the most obvious place to put a low-pass filter is just after audio capacitor C1. I've seen designs where a capacitor of 100pF was put in parallel with feedback resistor R6, but the value of such a cap is really critical and a value of 100pF is (including parasitic capacitances of circuit boards etc.) already too much.
First I will start with a proto setup, in which I can try-out the components and tune the design further.
As said before, this is the schema I will use for the amp.
For the final version of the amp I still have one of these steel cases left on the attick which I planned to use this time. But something came in between and the plan is off.
Today I went to one of those shops where they recycle stuff that people throw away. I found a nice old amp, a Pioneer SA_5300 which will be perfect for my purpose. It cost me 20 Euros. The advantages are clear over the new housing: It has all necessary holes drilled (but not too many) for volume control, power switches and input selection. The frame is much more rigid than the new steel box also and the knobs are fine!.
Of-course I will completely empty the Pioneer to make room for a tube, two toroids and input selectors close to the input section on the back.
First I started stripping the original Pioneer amplifier and took the board out. For the moment I will keep the AC inlet, and the cinch input connectors. The front plate can be removed without problems, so that potmeters and selector switches are accessible
The rest of the amp, including the PCB is nice for studying old amplifiers but does not add value to our project and will eventually be scrapped.
I
The following items need to find a place in the new amp:
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