Hmmm... Strike one? Let's try another post.So, let’s talk about the order of the elements in the signal chain. When dealing with a multistage signal chain with lots of elements, there are a few driving considerations. The first is gain control and signal swing. The second is distortion. And the third is noise.
Overview:Because I have four active fixed gain stages (buffer, gain recovery, driver, & power stage) and three variable gain stages (volume, tone stack, & master volume) in this amp I have to be especially mindful of gains and signal swings. Too large a signal swing in certain places can drive following stages into cutoff or compression before the power stage can reach full power. In general, smaller signal swings through the variable stages is preferred.
There is also a complication in the operation of the tone stack. Although the tone stack I’ve chosen (James-Baxandall passive) has a nominal design gain of ≈ -21dB, the gain can vary by as much as 38dB dependent on frequency and control settings. To prevent the inadvertent overdrive of later stages, the tone stack needs to be in the front end of the signal chain where voltage swings are small in the absolute sense.
The solution is to virtually divide the amplifier into two pieces. First a small signal section where gains are set, tonal variations are made, and overall output level set. And second, a traditional power stage which is voltage driven to produce the desired SPL at the speaker. The dividing line can really be on either side of the master volume control in my last post. Hence we are left with the low signal swing portion (i.e. the “preamplifier”) and the large signal swing portion (i.e. the “power amplifier”).
This approach also helps with distortion. Smaller swings in active stages in general means lower distortion. This means that the preamplifier can be made relative low distortion and the overall distortion can be controlled by the design of the driver and power stages.
Now the overall noise figure of the amplifier is largely set by the first gain stage which is the “gain recovery” stage. Before this, each of the stages represents a loss which hurts noise figure. This means we need to pay special attention to the first four stages so we can guarantee a nice quiet amplifier.
Design Decisions:As I said above, the goal is to have relatively low signal swings in the preamp sections. As such, I was willing to take a hit on noise figure and put the lossy elements (Gain controls, buffer, & ton stack) in front of the first amp to control that swing. During the build I will take extra care to keep the noise low prior to the first gain stage.
The gain recovery stage is just our old friend the 4S. I did this for two reasons. First, the buffer driven tone stack followed by a 4S has been proven to have excellent tracking and performance. Second, there are situations where it may be desirable to increase the gain a little and this topology also handles this eventuality particularly well as evidenced by these response plots.
Attachment:
TS Response Combined_s.jpg
The only other real choice was the positioning of the master volume control. This could go between the gain recovery and the driver stage, or between the driver and the power stage. The advantages of the former are lower distortion and greater control of the power stage operations. The latter would allow some tonal shaping of the output by the driver stage. I chose the former as the goal in this amp is tonal neutrality of the basic design and this placement of the master volume better supports that goal.
Gain Study:The next step was to look at the end-to-end gains and see if there are any problems. This required me to make some design decisions about the individual stages. For the preamp I decided to use the stages designed for the Baxandall circuit
here.
For the power stage I needed something that would cleanly drive my 6L6 SE-UL stage to its full bias of 32 volts. While investigating various drivers, I was perusing the back of my RCA RC-30 Receiving Tube Manual and noticed a recommendation in the “Resistance-Coupled Amplifiers” section using a 6SL7 with a 3.2kΩ self bias and a 220kΩ load. This as about a 33dB gain stage and has the added benefit of being biased closer to cutoff than conduction and hence is even order distortion biased at high drive levels.
So here is a gain summary spreadsheet with the assumed stage gains and stage bias numbers inserted for each stage as appropriate.
Attachment:
Gain Study - Nominal.png
In this example the input to the amplifier is assumed to be 2v-rms (2.218v peak). The gain controls are backed of by -10dB (≈ 2 o’clock) and the master volume is backed of by -20dB (≈12 o’clock). The tone stack controls are set flat with a nominal gain of -21dB.
In this example, the output level is about 16dB down from maximum output (still fairly loud) and there are no overages in the signal chain. However, looking at the tone stack plots above, it is possible to boost the high or low frequencies by ≈19dB above the nominal value. This means that if one was to turn up the bass or treble frequencies at the gain settings, somewhere near the highest settings, the power stage could be pushed into overdrive at those frequencies.
Here is a modification of the spread sheet showing the bass control turned fully clockwise.
Attachment:
Gain Study - Bass Boost.png
Here we see the power stage driven ≈2.7dB into overdrive. However, this assumes first, that the bass frequencies are at the same level as the rest of the music and second, that the gain controls are unmodified. However, by backing off either the gain controls or the master volume by only 3dB the problem is eliminated.
Attachment:
Gain Study - Bass Boost Corrected.png
This also demonstrates that with both the gain controls and the master volume set at “half volume” (i.e. 12 o’clock) there remains ample margin for even maximum bass or treble boost with out overdriving the power stage. This is shown below with the gains set as indicated and the tone controls flat.
Attachment:
Gain Study - Middle Settings.png
Here there is ample room for both volume increase and tonal shaping without overdriving anything in the signal chain.
Overall this is a very well behaved and balanced signal chain design. In the next post we’ll solidify the design and discuss some tube rolling options for this amp. As always, questions and comments are more than welcome.
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