~ Universal Amplifier / Buffer PCB ~

In addition to my QUAD 22 RIAA PCB which you may have seen here I also produced a flat response ~ triode based ~ "Line Stage" PCB with high input impedance and cathode follower low output impedance ~ The PCB is the same small format as the QUAD 22 RIAA PCB because it was also used to modify some QUAD QC22s to have a low output impedance for line driving while remaining "all valve"
The primary constraints for the original project were a small PCB footprint to fit the QUAD22 and NO semiconductors

The gain required was only ≈10dB (≈3.2x) and the output impedance needed to be lower than 2kΩ ~ The distortion spec. was 0.1% at 8dBu (≈2Vrms) output ~ Frequency response from 2Hz to 40kHz (at –3dB points) ~ Output load was not specified other than "long cables"

There are many ways to meet the above spec. and many were discounted due to overall complexity increasing the size of the finished layout ~ Use of a Pentode/Triode such the ECF80 looked like an option but the end user of the amplifier had many 12AT7 12AU7 and 12AX7 based products and carried these and ECC81–83 as spares so using one of these valve types became the first choice

Topologies like the µ–Follower and the now popular Shunt Regulated Push Pull (SRPP) were avoided as they both require a higher HT supply voltage than the 180V available to achieve the spec. ~ In practice both these arrangements also suffer from gain variation due to valve type and condition unless feedback is somehow applied

As the "boys own book of valve amplifiers" states ~ When using a triode valve the anode impedance ra restricts the gain and the maximum gain or µ is only achieved with a constant current anode load ~ The cathode must also be held at signal ground with either a large value bypass capacitor across any cathode resistor or by using a "fixed" bias scheme (designs that require batteries or multiple odd value supplies should be avoided)

Having achieved the maximum gain it should not be reduced by loading so a cathode follower can be used to buffer the gain stage from the output load ~ However in practice a specific gain is often required and this is achieved along with a reduction in distortion by use of overall negative feedback which may introduce instability and increases the complexity and size of the "design"

A simple way (not from the boys own book) to get a reasonably high and predictable gain with low distortion and low output impedance and without having to worry about overall d.c. and a.c. feedback calculations etc. is shown below ~ This circuit will provide 20dB or more wideband flat gain with a low output impedance

Above is the "schematic" of one channel of the Amplifier/Buffer PCB ~ V1a (pins 1–3) is the gain stage with the cathode resistor R30 not bypassed so it provides (acceptable) local negative feedback which reduces the distortion and increases the effective value of the triode internal anode resistance from ra to r+ µR+ Rk

The gain of a valve amplifier can be calculated if two of the values µ or gm or ra are known or can be determined from published data for the valve used ~ There are several ways this can be done ~ The traditional method of measuring from published 'curves' is very awkward when operating at low anode current or voltage as this amplifier does

The ECC81 as used above will be operating with an anode voltage of about 70V ~ The cathode is at 30V so Vak is only about 40V ~ Extrapolating from 12AT7 or ECC81 tables for Vak=100V gives gm≈2.6mA/V and µ>52 ~ For a lower Vak  µ will tend to be higher for the same Ia and the ra curve tends to "flatten out"

The ECC81 internal anode impedance ra for Vak ≈40V is increased by R=27kΩ to ra' and will be about 20k + (52 x 27k) + 27k   or   ra' ≈1.3MΩ .  .  .  see calculations from characteristic curves ~ The values of µ gm and ra may be lower than expected when operating at low Vak or a high Ia due to grid current ~ see ECC83 comments

The anode load RL' used for gain calculation is R23 100kΩ in parallel with ra' 1.3MΩ or RL'≈93kΩ ~ The reduced gain due to Rk feedback is ≈RL'/Rk or about 3.45x ~ In practice the gain after the cathode follower is about 3.2x or 10dB ~ You may think you have seen this circuit already but here V1a is not auto biased because Rk is far too high

In a normal auto bias arrangement Rk would be a lower value and in many circuits would be bypassed by a large electrolytic capacitor but in this circuit the grid of V1a is raised +ve by R22 to prevent the reduction in Ia due to the large value of Rk ~ Resistors R22 or R29 or both can be adjusted to set the optimum voltage at the anode of V1a

A way to view the gain of V1a is that the input voltage at the control grid appears across the cathode resistor due to 'follower' action so the mutual conductance gm of the valve with Rk is a lower value gmk ≈ 1/Rk and the gain Av = gmkxRL' = RL'/Rk ~ For higher gains by using lower value Rk the total RL' is also reduced by ra' not being increased so much but it will still be much higher than the valves native ra

For higher gains >15dB the ratio RL/Rk should be adjusted to be higher than calculated ~ The amplifier signal path requires no high value or electrolytic capacitors apart from the external PSU decoupling and possibly across C15 for an extended Bass response ~ The input impedance is set by R34 R29 and R22 and could be made higher as the parallel combination of R22 and R29 can be the manufacturers maximum g1 resistance for auto bias

The PCB for this project used small MRS25 0.6W resistors which places a limit on maximum Ia of the cathode follower V1b ~ With about 50V across R32 the current could be made as high as 10mA ~ The PCB can be used with supplies up to 350V and R32 could be a 1W or 2W resistor mounted on end if a higher V1b cathode voltage and Ia are required
Note: The input and output capacitor pads can accept a number of capacitor types both inline and across the PCB footprint and are configured to allow parallel capacitors so electrolytics with polycap bypass can be fitted if high C values are required ~ The heater connections can be configured for series or parallel enabling use of valves like E88CC or 12V battery supplies for ECC81–83 if required

Like my Cascode driver for the QUAD II this amplifier topology was developed during 1980s but not made into a PCB at that time ~ For this project I decided to go straight to PCB layout without breadboarding as I knew the design would meet the spec. ~ The only drawback ~ if there is one ~ is that very good HT smoothing and regulation may be required but then good amplifiers should have this anyhow

Like my QUAD II Cascode driver and the QUAD 22 RIAA PCB these PCBs are available to purchase either as plain PCBs or as a kit of parts including valve bases and mounting hardware but not the resistors and capacitors which will be determined by your design and the HT and gain expected for the valves used

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