Classic. Construction and User Guide. Ian Thompson-Bell

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1 Classic Construction and User Guide Ian Thompson-Bell Version 1.1 February 2015

2 Introduction The Classic is one of the boards that make up the EZTubeMixer family. It can be used as part of an EXTubeMixer, in the LunchBox project or on its own in a 19inch rack mounting enclosure for instance. Its design is based on one of my earliest microphone preamplifier designs hence the name. When I first began to design tube microphone preamps I was very aware of the stability limitations of negative feedback (NFB) in vacuum tubes, so I looked for ways of obtaining reasonable gain, drive capability and low distortion without using NFB. After a lot of research I eventually decided upon the mu follower topology and the 6CG7 tube. The mu follower topology achieves a distortion level close to the intrinsic distortion of the chosen tube and the 6CG7, being a 6SN7 in a B9A envelope, has one of the lowest intrinsic distortions of any tube. A 6CG7 mu follower stage achieves about 26dB gain and will drive +28dBu into a 10K load. Two such stages, with a level pot between them, and a 1:10 mic input transformer, can give an overall gain of close to 70dB and in EIN below 120dBu. This was the basis of my first mic pre. There are two problems with the 6CG7 mu follower design. First, it will only drive a 10K load so there is no way it can drive a 600 ohm load without seriously compromising its headroom. Secondly the tube is rather tall too tall to fit into the EZTubeMixer module format. The drive problem is solved by changing the tube to a This has a lower plate resistance and with sufficient idle current can drive a 2400 ohm load. I had designed, built and tested this some time ago to provide transformer balanced direct outs for a mixer design and its distortion performance proved to be remarkably similar to the 6CG7. With a 2400:600 transformer it would drive +20dBu into 600 ohms (1% THD). Fortunately, by using a 6922, the tube height problem is also solved as the 6922 is already used in the EZTube Mixer project so this design was ideal for the Classic. Schematic The schematic of the classic is shown on the next page. The partitioning follows the same scheme as the Eurochannel PCB. Two identical amplifiers are provided, one connected, via a relay to a microphone input transformer and the second one uncommitted. The relay provides the possibility of a HIZ input. The standard EZTubeMixer 32 way connector is used for inputs and outputs so OUT1 and OUT2 are connected to the same pins as in the Eurochannel. The Eurochannel stepped gain control, and buttons for phantom, pad, phase and mic/line switching are NOT included. Instead, space for a second input transformer is provided (see Configuration for details). Phantom power decoupling is provided and phantom power is brought out to two pads on the PCB.

3 Although it only uses two tubes, the classic consumes more HT current than the Eurochannel. Each tube idles at approximately 10mA giving a total HT current requirement of 20mA. You need to take this into account when designing the HT supply. Configuration The classic can be configured in a number of ways: High Gain Two Stage Mic Pre This uses a microphone input transformer for T1 which is connected to the first amplifier.the PCB is laid out for the following microphone transformers: Sowter 1948c Cinemag CMMI 10PCA Jensen JT 115K EPC The first amplifier output, OUT1, is fed to a 10K LOG gain pot and then to the IN2 of the second amplifier (transformer T2 is not used). The output is taken from OUT2. An external 2400:600 transformer provides a balanced output. The input of the microphone transformer can be connected direct to the mic input from the 32 way connector. In this

4 configuration 70dB of gain is available. The amplifier is suitable for ribbon or dynamic microphones. The facilities of this simple mic pre can be extended in a number of ways. Firstly, simple EQ can be added before the gain pot. A simple high pass filter can be added using a couple of capacitors and a centre off SPST toggle switch as shown in the schematic below: In the Flat position, C1 and C2 are shorted. In the centre position (80Hz) a HPF is formed by C1 and the 10K gain pot. In the lower position (40Hz) C2 is added in parallel to C1 lowering the HPF frequency. This simple HPF provides a slope of 6dB per octave (see measured response graphs below).

5 The second upgrade is to add phantom, pad, phase and mic/line switches. To provide these, a small PCB containing four toggle switches is available. The schematic of this PCB is shown on the next page: The mic and line inputs are on the left. They are on 3 pin Molex KK connectors on the PCB. Phantom power can be switched to the mic input via S1. The phantom power input is a pad on the PCB. A 20dB pad can be switched in via S2. This pad can be changed to 30dB by changing one resistor. This is useful for the classic which has about 48dB of gain in the first stage (20dB from the transformer and about 28dB from the 6922 tube) which means it could run out of headroom with exceptionally large input signals. The line input is fed via a 33dB attenuator before going to the mic/line switch S3. Again, for the classic this needs to be increased. Changing R7 and R8 to 10K increases the attenuation to 42dB. A +4dBu input now becomes +9dBu at the output of the first stage. As the mu follower is capable of an output level of +30dBu into 10K there is still 21dB of headroom. Finally, the output of the mic/line switch feeds the phase switch S4. This is slightly different to the Eurochannel and allows the phase of the line input the be flipped if required (on the Eurochannel only the mic phase can be flipped). The output is once again on a three pin Molex KK connector. The four toggles PCB is shown on the next page:

In this case the four toggles are mounted on the same side as the other components. The Molex KK connectors are mounted on the back of the PCB.

6 On the left is the bare PCB. This contains two complete four toggle circuits. The dotted line down the centre separates the two. To use one circuit on its own you just saw the PCB into two along the dotted line. In the centre is a half PCB built. In this case the four toggles are mounted on the same side as the other components. The Molex KK connectors are mounted on the back of the PCB. It is probably better to mount the toggles on the rear of the PCB and the Molex connectors on the component side. That way you can read the legend next to the connectors when you hook up the PCB. The picture below shows the four toggles PCB mounted to a front panel and wired to a classic PCB.

7 The picture below shows a completed classic from the front incorporating the four toggles PCB and the high pass filter:

8 Twin Mic Pre The second configuration of the classic board is as two mic preamps. In this case another 1:10 microphone transformer is fitted at T2 as well as the one at T1 to provide two mic inputs. The input of one is wired to the mic input of the 32way connector and the other is wired to the line input on the 32way connector. The output of the first mic pre is from OUT1 and of the second is from OUT2. If external 2400:600 transformers are fitted then each mic pre has a maximum gain of 42dB (20dB from the input transformer, 28dB from the amplifier less 6dB step down in the output transformer). In this form it is similar in performance to the V72s except it has a little more gain. To vary the gain, the simplest method is to connect the microphone transformer secondary to a 100K LOG pot and connect the wiper the the amplifier input. The maximum input level is now limited by the performance of the input transformer; the amplifier will never be overloaded. If you want to be able to drive the tube harder to get added tone then you should add a 600 ohm balanced attenuator after the output transformer. Either of the two circuits shown below could be used: You can also use the four toggles PCB with the twin mic pre. In this case there is no need to saw the PCB in half as you need both halves. As the preamp gain is lower, there is no need to increase the attenuation of the 20dB mic pad or the line input pad. unfortunately there are not enough spare pins on the 32 way connector for the two additional line inputs. You could use the FG and FS pins for one of them but all the other spare pins are buses (MB1 thru 4 for example). If you are using the EZTubeMixer motherboards and you are prepared to cut a few tracks on the motherboard then you could use a couple of the MB pins for the other line input. If you are not using the motherboard (e.g building it into a 19 inch enclosure) then you can use any pins you like.

9 Two Stage Mic Pre with passive EQ The two stage mic pre can also be use with almost any passive EQ design. the first stage has a low enough output impedance to drive most passive EQs and the second stage acts as gain make up for the passive EQ. The overall gain will as for the two stage version less the insertion loss of the passive EQ. So for example, if the EQ insertion loss is 20dB then the overall gain will be approximately 50dB. The level control should be placed after the EQ. Its value should be chosen so as not to load the EQ output. Twin Poor Man s Tube Gain Make Up The classic is basically a pair of poor man s tube gain make (PMTGMU) up stages on a single PCB. it can therefore be used anywhere that the PMTGMU is used. It is typically used as gain make up for a passive EQ. In this configuration, 10K:10K line input transformers would be used, the secondaries of which would be connected to the input of the EQ. The PCB is laid out for the following 10K:10K line input transformers. Sowter 3293 Cinemag CMLI 15/15PCA Jensen JT 11P 1HPC The output of the EQ is then fed to the amplifier inputs. Most passive EQs have an insertion loss of less than 20dB. The 28dB gain of the classic amplifier is enough to make up this loss and the 6db loss in the output transformer. A simple preset pot can be used at the EQ output to set the overall gain to unity.

10 Construction Like the Eurochannel, most of the component values are silk screened on the PCB to ease assembly. The PCB layout is shown below. Start with the smallest components, the tube pins, followed by the resistors. Note that the two resistors labelled 6922 are zero ohm links that need to be fitted when 6922 tubes are used. It is possible to use 6CG7 tubes with this PCB in which case the links should be omitted. Then add the film capacitors and the electrolytic capacitors. Don t forget the Hlink wire else the tubes will not light up. Lastly fit the input transformer(s). BOM This section lists the components required to build the classic PCB. Like the Eurochannel, most of the passive component values are printed on the PCB silk screen to ease assembly. There is a minor error on the V2 PCB silk screen where some resistors are labelled 680R instead of 220R. If you have a V2 board just fit 220R resistors wherever you see 680R on the silk screen.component idents are used only for the tubes, transformers, the 32way connector and some links. The components fitted to the places marked 6922 are described earlier in this document. Note: all resistors are 0.25W metal film unless otherwise stated. Where special components are used the Farnell part number is given.

11 Ident Component V1, V mm tube pins (18) Farnell # T1 T2 J1 see text see text 32 way connector (see text) RY1 NEC EA2 12NU Farnell # There are several alternatives for the 32W connector. The two cheapest are the following Farnell part numbers: , Resistors Value Quantity 100R 1 220R 4 680R 2 4K K 2 220K 2 220K 1W RHT 1Meg 2

12 Capacitors Value Quantity 100nF 400V metal Film 4 (Farnell # ) 100nF 400V Metal Film CHT (Farnell # ) 4.7uF 400V Metal Film 2 (Farnell # ) 47uF 100V Electrolytic 5mm lead pitch 1 (Farnell # ) 220uF 63V Electrolytic 5mm lead pitch 1 (Farnell # ) Note, the Farnell part numbers given above for capacitors are just suggestions. There are many other parts that meet the specification.

13 Version Control V1.0 Initial issue V1.1 added TLY to BOM. Added note about silk screen error in V2 PCB

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