Fullscreen HTML5Med Res (??MB)HTML4

Modifications to the 16-channel mixer In February, March, April & May of this year we published a 16-channel mixer design which has been very well received by enthusiasts. Predictably, many people are modifying the design to suit their own purposes and equally predictably, some have found that the performance can he improved. Whenever we publish a new design, we do so in the expectation that at least some of our readers will closely check through the design parameters to see what it achieves, to see if there are any mistakes and to see if it can be improved. And every now and again, some of these readers are moved to tell us the results of their endeavours. One such reader is Phil Denniss and we'll let him take up the story: I was filing away my back issues of SILICON CHIP when I came upon the articles for your 16-Channel Mixing Desk that were printed earlier this year. As I have a great interest in audio electronics I decided to read through the articles. Before pursuing this much further I would like to say that while I have not read the entire set of articles, I found the presentation very good, particularly the setting up procedure. But it is a shame to see such a lot of effort messed up by a fairly small but important mistake. However, the situation is very easily fixed without any need to change the circuit board. Basically, the problem is that the resistor values chosen do not yield the specified differential input impedance, nor do they give very good common mode rejection with any S1 : 1 : MIC 2 : LINE BALANCED 3 : LINE UNBALANCED 10DpF 10k 1°/, 1k 1% MIC ZO-....-'r--......- - - l l t - -......---w,1,,----...,_-"1 AND LINE INPUT 30---'-::+---+---tlr--4t----"WiAc--+----t--.......- - - - : : I S1b 2,~ _. • DENOTES CHANGE FROM ORIGINAL CIRCUIT "1' ia'l 0 REVISED PREAMPLIFIER FOR 16-CHANNEL MIXER Fig.1: revised circuit for the 16-Channel Mixer preamplifier stage. The component values that have been changed are highlighted with a star. Compared to the original, this circuit offers better common mode rejection and therefore is less prone to hum & noise pickup. source that does not have zero source impedance; ie, most real sources. I have done some calculations to establish what the input impedance and common mode gain are for the general case using the published circuit. I checked the results by lashing up the circuit and measuring the relevant circuit parameters. I initially used an LF351 op amp at DC without coupling capacitors because it was easier for me. I found that the 9H2 resistor should be 1kQ and the 427Q and 483Q values should total 10kQ, to balance the values in the inverting arm of the circuit. This will yield an input impedance of 1.07kQ and give a common mode gain determined by the matching between the resistors around this stage and the balance in the source resistance. I reckon there should be a 100pF capacitor to ground from the non-inverting input of the op amp as well, to keep a lid on the common mode gain at high frequencies, and I feel that the 10kQ resistors just after the input coupling capacitors do not do anything worthwhile either, though I might be persuaded otherwise. Initially, when considering the noise performance using the suggested modifications, I reckoned that maybe it would be 6dB worse because the input resistance was roughly doubled, or perhaps 3dB because the noise currents in the two op amps were perhaps correlated (it turns out that they are not). Well anyway, I figured that I had better check it out properly with an LM833, to find out the real answer. It sure surprised me. I figured that with the inputs shorted to ground, the difference in S/N between the two circuits would be less than 1.5dB. With 300Q to ground on each input (ie, a 600Q source), the difference would be about 1.15dB. This is because the input noise depends hardly on the NOVEMBER1990 75 11le MB3 I•• n»dium to ,,.•vy duty two pl•tfonn mounting br•ckel spec/flc.lly de•igned for •curing •peak,,n, ate lo wall• and eel/Ing•, deab or bench tape. Once mounted lhe br•cket can be rotated• full 360 degree• u _, u l»lng •wfve/led up or down until lhe required viewing or ll•tening •ngle I• •che/ved. I~ Both pl•lfor1T111 have predrllled hole• for mounting and •II moving parla and jolnta are eaai/y re/eaaed or locked with an large a/Ian key aupplied with lhe unit ~ ELECTRONICS CHRISTIAN BLIND MISSIQ~ ·J R,,gtoriKf[;~~e, bt1« COUPON Please cut and send to: CHRISTIAN BLIND MISSION INTERNATIONAL. P.O. _Box 5, 1245 Burke Road, KEW. Vic. 3101 Phone: (03)817-4566 e D Please send me further information about CBMl's work. As long as it is possible for me. I will help: D monthly D quarterly D annually D lo prevent blindness CJ to restore eyesight CJ to rehabilitate the blind Enclosed is my gift of$ _ _ _ _ __ Mr1Mrs, Miss, Street, __ _ City : __ . . l'aslcode : The MB5 i• • heavy duty IWO platform mounting bracket de•lgned for securing alTIIIII TV'• and speaker• lo walls, ceiling3, desks or bench Iopa. When mounled It can be rotated• full 360 degrees •• -11 n being swlw,lled up or down to any viewing or listening angle. Both p/sl -forlTIII have predrilled holes a'D , ~ .n.&\.liJ ,I ft ELECTRONICS 76 for mounting and sreenily adjusted with •large •llen keysupplledwlthlheunit. SILICON CHIP noise current of the op amp at all but depends mostly on the input noise voltage and to some extent on the bias and feedback resistors. While studying the LM833 data, I noted a graph of "Input Referred Noise Voltage vs Source Resistance" and this plot clearly shows that the noise does not rise 6dB above the zero Rs level until the source resistance exceeds 3kQ. For my circuit this does not happen until the source resistance actually reaches lkQ. At this value of source resistance, the original op amp will start to seriously load the source and this will in turn increase the SIN ratio more than my proposed circuit will. Just to make sure, I decided to lash up the circuit and measure the noise. The results were not too satisfactory because I used a protoboard which left the circuit open to pick up noise and hum. The exercise mostly showed how quiet the LM83-3 is and how easy it was to pick up a lot af unwanted garbage. However, it did show that the equivalent input noise voltage was about lµV and that the difference in noise performance between the two circuits is very small. Generally the published circuit is not a very good performer because of this very problem. The performance of the preamp is too dependent on the source impedance and the source impedance presented to the preamp is unknown. The ideal solution to this is to use a circuit that provides much higher input impedance, such as the two op amp instrumentation amplifier, so that the source impedance has much less effect. This will change the noise performance of the preamp. At worst, the noise may increase by 3dB but it may provide better performance because the preamp will effectively see a lower source impedance. It will see only the impedance of the source and not the input resistors. Another less important point I would like to discuss is the extensive use of electrolytic capacitors for coupling. I appreciate that circuit designers are pretty well up the creek when it comes to good high value (470nF and upwards) capacitors, and it is very hard to find an acceptable alternative to the old electro in terms of size, cost and capacitance. Bipolar electros seem to have disappeared. I think it would have been safer not to use so many electros, although this may compromise frequency response and/or noise performance. But more importantly I think you have specified the incorrect polarity for some of the coupling capacitors in the mixer and this may degrade the performance of the mixer somewhat. My reasoning is this: the LM833 has a fairly high input bias current, 500nA typical and lµA max according to the National book, and this can produce quite a high offset voltage (5m V or more) at the inputs of the op amp. This is amplified by the op amp (if it has a DC gain of more than one) and can reach 100mV or so at the output. Now the input transistors of the LM833 are PNP (for low noise I guess) so the current flows out of the input and will therefore produce a positive voltage across the input biasing resistor, or feedback resistor for the inverting input. It is important to note that the offset so caused may be positive or negative at the output, depending on the configuration of the stage, but is usually positive at the input. In the mixer, nearly all the electrolytic coupling caps have their positive terminal connected to the source and their negative terminal connected to the load. A quick check over the circuit indicated that the input capacitors of ICs la, 2b, 6a, 6b, 7b, 8b & 9b, and the output capacitors for ICs la, lb, 2a, 7a, 8a & 9a, are the wrong way around. I cannot tell the extent to which this will affect the performance of the mixer but I think that it is advisable not to reverse bias electrolytic capacitors if possible. (P. D., Chippendale, NSW). • How do you answer an onslaught such as this? Well, as we have done, you publish it. We quite agree that our single op amp balanced input will not give as good common mode rejection as the classic twin op amp design: In designing the balanced input stage for the mixer, we were deliberately trying to minimise the op amp count and obtain the best signal/noise ratio. However, as Phil Denniss points out, the circuit can be modified to improve the common mode rejection, particularly with real source impedances such as 600Q, without a significant increase in the residual noise. We calculated the noise performance for both single op amp circuits Fig.2 (left): this is the amended wiring diagram for the preamplifier board. The changes are all at the top of the board, near switch S1. and confirmed that the suggested modifications will provide only a 1.3dB increase in noise while changing the common mode rejection from -16.2dB for our circuit to better than -48dB with the modifications. We also tested the alterations and found the S/N ratio to be -99dB with respect to a 2V output and 600Q source impedance. The original arrangement produced -90dB under the same conditions, although this measurement was masked by hum pickup. The results indicate that Phil's modifications give superior results since the hum pickup is considerably less due to the improved input balance. Making the changes The revised circuit for the preamplifier is shown in Fig.1. By comparing this to the original circuit on page 61 of the March 1990 issue, you will see where the changes are. To help you spot the changes, we have highlighted each changed component value in Fig.1 with a star. Most of the circuit changes are associated with the section involving pin 3 of ICla. To help those who have already built the mixing desk, or those who intend to build it, we have produced an amended wiring diagram for the preamplifier board - see Fig.2. This will take the place of the wiring diagram shown on page 72 of the April 1990 issue. Note that the wiring to the switch is now simplified. The shielded cable from the pole of Slb to point "x" on the original diagram has been removed entirely. So has the shielded cable from the very top of the switch wafer. If you intend making changes to the original switch wiring, follow the new diagrams very closely. Six components along the switch end of board are altered. If you look at the top of the board you will see a line of 7 components, with two 33µF capacitors at the end. The changes to these are tabulated below: The 91Q resistor adjacent to these components is also Old 1.2kQ 10kQ 560Q 1.8kQ 470Q 13Q 0 New 1.1kQ 10kQ 100pF 10kQ link link changed, to lkQ. Note also the 1. lkQ resistor added across the switch and the earth from point 12 on the switch to the board. In addition, as noted by Phil Denniss, a number of electrolytic capacitors in the circuit are reversed in polarity. These are now correctly shown on the diagram of Fig.2. However, capacitors on the other boards should also be reversed. The capacitors in question are the input capacitors of IC2b, IC6a, IC6b, IC7b, IC8a & IC9b and the output capacitors of IC2a, IC7a, IC8a & IC9a. Ideally, these capacitors can all be bipolar types which are readily available, although they do cost a little more.~ NOVEMBER 1990 77