<html><head></head><body><div class="ydp6f59649ayahoo-style-wrap" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;"><div></div>
<div dir="ltr" data-setdir="false">blah i wasn't finished with my email :)</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">2. phase response at low frequencies sometimes matters more than amplitude</div><div dir="ltr" data-setdir="false">3. tolerance of Ecaps can be totallly shit</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">I haven't had the courage to use polarized Ecaps in situations of +/- DC offset (even small). I should probably investigate more as others here have suggested it's probably OK.</div><div><br></div>
</div><div id="yahoo_quoted_5985289825" class="yahoo_quoted">
<div style="font-family:'Helvetica Neue', Helvetica, Arial, sans-serif;font-size:13px;color:#26282a;">
<div>
On Wednesday, February 16, 2022, 11:56:55 AM EST, ben gebhardt <benjamin_gebhardt@yahoo.com> wrote:
</div>
<div><br></div>
<div><br></div>
<div><div id="yiv0056098690"><div><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;" class="yiv0056098690ydpeec38fddyahoo-style-wrap"><div></div>
<div dir="ltr">I've used back to back Ecaps plenty (lots out in the field) with no issue. I used to bias them but talked myself out of the biasing being necessary.</div><div dir="ltr"><br clear="none"></div><div dir="ltr">I've used DC servos alot also and this can be an attractive solution depending on the circuit.</div><div dir="ltr"><br clear="none"></div><div dir="ltr">I'd argue that low corner frequencies are desirable for a number of reasons (circuit depending of course):</div><div dir="ltr"><br clear="none"></div><div dir="ltr"><ol dir="ltr"><li>cascaded stages can start to significantly eat away at the low frequency response</li></ol></div><div><br clear="none"></div>
</div><div id="yiv0056098690yqt28930" class="yiv0056098690yqt3118364767"><div id="yiv0056098690yahoo_quoted_5596351514" class="yiv0056098690yahoo_quoted">
<div style="font-family:'Helvetica Neue', Helvetica, Arial, sans-serif;font-size:13px;color:#26282a;">
<div>
On Wednesday, February 16, 2022, 11:41:02 AM EST, Steve Lenham <steve@bendentech.co.uk> wrote:
</div>
<div><br clear="none"></div>
<div><br clear="none"></div>
<div><div dir="ltr">I have no experience with a real 303, but looking at the schematic it is <br clear="none">easy to see why that would be the case.<br clear="none"><br clear="none">C29 smooths the bias voltage rail for the differential pair on the <br clear="none">output of the VCF. That rail biasses the two diff-amp transistor bases <br clear="none">but also sets their tail current.<br clear="none"><br clear="none">If C29 leaks, the bias rail will drop and so will the tail current. And <br clear="none">a diff-amp with a variable tail current is actually a very crude sort of <br clear="none">VCA. So lower bias voltage = lower gain for the diff-amp = lower output <br clear="none">from the VCF = not so much signal to feed back to generate resonance.<br clear="none"><br clear="none">Cheers,<br clear="none"><br clear="none">Steve L.<br clear="none">Benden Sound Technology<br clear="none"><br clear="none"><div id="yiv0056098690yqtfd10928" class="yiv0056098690yqt9297468333"><br clear="none">On 16/02/2022 12:35, <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:rburnett@richieburnett.co.uk" target="_blank" href="mailto:rburnett@richieburnett.co.uk">rburnett@richieburnett.co.uk</a> wrote:<br clear="none">> Yes, I totally agree. Out of all the places where they might have put <br clear="none">> one of the electrolytics in backwards, this is one place where you <br clear="none">> wouldn't expect them to have got away with it!<br clear="none">><br clear="none">> I had a bit of a play about at the time when I discovered this... <br clear="none">> Replacing the cap with a new one fitted either way around had the <br clear="none">> effect of making the filter sound more slightly more resonant and <br clear="none">> "shrieky" particularly at the high-frequency end. But that could just <br clear="none">> be a new vs old capacitor thing, or even just down to capacitance <br clear="none">> tolerance. I never got round to actually measuring C and ESR of the <br clear="none">> old pulled cap though, before I put the original cap back in (the <br clear="none">> wrong way around again of course) and closed up the box. Those old <br clear="none">> phenolic PCBs don't put up with much soldering/desoldering :-(<br clear="none">><br clear="none">> My impression is that leakage current through the cap fitted the wrong <br clear="none">> way round was decreasing the loop gain around the filter and making it <br clear="none">> less resonant. So it might account for the fact that the TB-303 <br clear="none">> filter never quite manages self-oscillation without some other <br clear="none">> modification. But as you said, it would be a strange way to guarantee <br clear="none">> this across a production run and for the lifetime of the product!<br clear="none">><br clear="none">> -Richie,<br clear="none">><br clear="none">><br clear="none">><br clear="none">><br clear="none">><br clear="none">> On 2022-02-16 11:57, Steve Lenham wrote:<br clear="none">>> That's less an endorsement of playing fast and loose with<br clear="none">>> electrolytics than a case of marvelling at how they got away with it<br clear="none">>> at all!<br clear="none">>><br clear="none">>> C29 in conjunction with R116 (10K) is acting as a lowpass filter,<br clear="none">>> decoupling the 5V bias voltage being fed to the VCF.<br clear="none">>><br clear="none">>> If R116 was much lower in value - or not there at all - how long do we<br clear="none">>> think C29 would last?<br clear="none">>><br clear="none">>> The unpredictable reverse leakage current through C29 probably means<br clear="none">>> that the bias voltage varies quite a bit from unit to unit and perhaps<br clear="none">>> with other things like temperature too. By all means claim<br clear="none">>> retrospectively that this contibutes to the magic of the TB303 but I<br clear="none">>> would be amazed if it were deliberate and it isn't good engineering.<br clear="none">>><br clear="none">>> Ah, you might say - good engineering doesn't matter if it sounds<br clear="none">>> right. But what we don't know is how many units got binned because the<br clear="none">>> leakage was so bad that the VCF didn't work at all...<br clear="none">>><br clear="none">>> Cheers,<br clear="none">>><br clear="none">>> Steve L.<br clear="none">>> Benden Sound Technology<br clear="none">>><br clear="none">>><br clear="none">>> On 16/02/2022 10:42, <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:rburnett@richieburnett.co.uk" target="_blank" href="mailto:rburnett@richieburnett.co.uk">rburnett@richieburnett.co.uk</a> wrote:<br clear="none">>>> There is at least one polar electrolytic capacitor in the Roland <br clear="none">>>> TB-303 that is fitted backwards and has significant average voltage <br clear="none">>>> across it (the wrong way!)...<br clear="none">>>><br clear="none">>>> <a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://www.matrixsynth.com/2010/11/component-c29-is-backwards-on-all-tb.html ">https://www.matrixsynth.com/2010/11/component-c29-is-backwards-on-all-tb.html </a><br clear="none">>>> The polarity of the capacitor is correct on the schematic in the <br clear="none">>>> service manual, but the PCB ident is the wrong way round. The actual <br clear="none">>>> component is fitted as per the PCB ident, not the schematic!<br clear="none">>>><br clear="none">>>> -Richie,<br clear="none">>>><br clear="none">>>><br clear="none">>>><br clear="none">>>> On 2022-02-16 10:00, Steve Lenham wrote:<br clear="none">>>>> just because we can "get away" with using standard polar electrolytics<br clear="none">>>>> (and we often can) doesn't mean that it is good engineering or that<br clear="none">>>>> there will be no negative effects under any circumstances.<br clear="none">>>>><br clear="none">>>>> Polar electrolytics will withstand something like half a volt of<br clear="none">>>>> reverse polarity with no ill effect. Above that, they start to leak<br clear="none">>>>> current, and if that current is not controlled then they will suffer<br clear="none">>>>> damage up to and including exploding. We've all seen what happens to<br clear="none">>>>> PSU caps accidentally fitted backwards.<br clear="none">>>>><br clear="none">>>>> In the case of audio coupling caps, the cap forms a single-pole<br clear="none">>>>> highpass filter with the input resistance of the stage it's feeding,<br clear="none">>>>> i.e. a reactive/resistive potential divider. At frequencies well above<br clear="none">>>>> the corner frequency of the filter, virtually all of the voltage<br clear="none">>>>> appears across the resistance. Great, no voltage across the cap (in<br clear="none">>>>> either direction!). But what happens as the frequency drops towards<br clear="none">>>>> the corner frequency? More and more of the voltage appears across the<br clear="none">>>>> capacitor until eventually the negative signal peaks cause the cap to<br clear="none">>>>> leak current. At that point you will start to get signal distortion.<br clear="none">>>>> The cap will rarely be damaged because there is usually enough<br clear="none">>>>> resistance in the circuit to limit the current to a safe level, but<br clear="none">>>>> fidelity might be.<br clear="none">>>>><br clear="none">>>>> That is why, as previously mentioned, you see bipolar electrolytics in<br clear="none">>>>> speaker crossovers. The corner frequencies of the filters in a<br clear="none">>>>> crossover, by definition, are located in the middle of the audio band,<br clear="none">>>>> so there will be substantial signal voltage across the capacitors at<br clear="none">>>>> all times. In addition, a speaker is usually a low-impedance system,<br clear="none">>>>> so there is nothing to limit any reverse leakage currents to a safe<br clear="none">>>>> level. Polar electrolytics would be a disaster there.<br clear="none">>>>><br clear="none">>>>> So in a nutshell, polar electrolytics for coupling are fine as long as<br clear="none">>>>> you can be sure that there will never be any substantial signal<br clear="none">>>>> content below the corner frequency of the filter that the cap forms<br clear="none">>>>> with the input impedance of the stage it's feeding.<br clear="none">>>>><br clear="none">>>>> Of course, this raises the question of what the cap is doing there at<br clear="none">>>>> all. If the above condition is met, the cap isn't doing any highpass<br clear="none">>>>> filtering. And it can't block unpredictable DC offsets (i.e. ones that<br clear="none">>>>> could be of either polarity) greater than half a volt. The answer is<br clear="none">>>>> that it is generally blocking small opamp offset voltages (of the<br clear="none">>>>> order of tens of millivolts) and preventing their being amplified by<br clear="none">>>>> subsequent gain stages.<br clear="none">>>>><br clear="none">>>>> Cheers,<br clear="none">>>>><br clear="none">>>>> Steve L.<br clear="none">>>>> Benden Sound Technology<br clear="none">>>>><br clear="none">>>>><br clear="none">>>>> On 16/02/2022 06:30, Brian Willoughby wrote:<br clear="none">>>>>> While writing firmware for a digital mixer, the folks over in the <br clear="none">>>>>> digital-controlled-analog-preamp team discovered distortion that <br clear="none">>>>>> was traced to the polarized capacitors. I did not look at the <br clear="none">>>>>> schematic, so I don't know what the exact problem was. They <br clear="none">>>>>> changed the capacitors for the next prototype and got rid of the <br clear="none">>>>>> distortion. I don't remember whether they switched to bi-polar <br clear="none">>>>>> caps or just found polar caps with a higher voltage rating. I seem <br clear="none">>>>>> to recall it was the latter, but it stuck in my mind that the <br clear="none">>>>>> electrolytics could cause distortion due to their polarized nature <br clear="none">>>>>> - at least for large input signals and/or high gain settings.<br clear="none">>>>>><br clear="none">>>>>> Sorry for the non-answer. Seems like there's definitely the <br clear="none">>>>>> potential for problems that can't be ignored, but the solution is <br clear="none">>>>>> not necessarily bi-polar.<br clear="none">>>>>><br clear="none">>>>>> Brian<br clear="none">>>>>><br clear="none">>>>>><br clear="none">>>>>> On Feb 15, 2022, at 12:30, Mattias Rickardsson wrote:<br clear="none">>>>>>> Hej Danjel and others,<br clear="none">>>>>>><br clear="none">>>>>>> I don't recall seeing any hard facts about non-polarized <br clear="none">>>>>>> electrolytics performing better than ordinary polarized in audio <br clear="none">>>>>>> circuits, but still they occasionally turn up in designs. Would <br clear="none">>>>>>> be interesting to hear why they sometimes are preferred by audio <br clear="none">>>>>>> designers, though! :-)<br clear="none">>>>>>><br clear="none">>>>>>> I searched for "non-polarized" in Douglas Self's reference book <br clear="none">>>>>>> "Small Signal Audio Design" and found a couple of applications <br clear="none">>>>>>> where they actually do make sense - but it's a practical reason <br clear="none">>>>>>> rather than an audio performance reason:<br clear="none">>>>>>> DC blocking in inputs & outputs, where it's possible that the <br clear="none">>>>>>> connected gear pulls the voltage way off ground level, and you <br clear="none">>>>>>> never know in what direction.<br clear="none">>>>>>> Douglas Self writes:<br clear="none">>>>>>><br clear="none">>>>>>> "C2 is a DC-blocking capacitor to prevent voltages from <br clear="none">>>>>>> ill-conceived source equipment getting into the circuitry. It is <br clear="none">>>>>>> a non-polarized type as voltages from the outside world are of <br clear="none">>>>>>> unpredictable polarity, and it is rated at not less than 35 V so <br clear="none">>>>>>> that even if it gets connected to defective direct-coupled <br clear="none">>>>>>> equipment with an op-amp output jammed hard against one of the <br clear="none">>>>>>> supply rails, no harm will result."<br clear="none">>>>>>><br clear="none">>>>>>> Any other good reasons for using them?<br clear="none">>>>>>> And sorry, no - I don't have any better advice than what has <br clear="none">>>>>>> already been said. :-)<br clear="none">>>>>>><br clear="none">>>>>>> /mr<br clear="none">>>>>>><br clear="none">>>>>>>> On Feb 15, 2022, at 11:32 AM, Mike Bryant wrote:<br clear="none">>>>>>>>> I've never bothered, I've never noticed a difference in just <br clear="none">>>>>>>>> using a 47μ instead of playing about with back-to-back <br clear="none">>>>>>>>> capacitors even after the thick end of 40 years the earliest <br clear="none">>>>>>>>> stuff I built that way still has capacitors that capacitate <br clear="none">>>>>>>>> just fine.<br clear="none">>>>>>>> -- <br clear="none">>>>>>>>> Gordonjcp<br clear="none">>>>>>>> Agreed. Unless you actually have a reverse DC bias voltage (in <br clear="none">>>>>>>> which case rotate the capacitor) I've never understood any need <br clear="none">>>>>>>> for the non-polarised capacitors. Most mixing consoles are full <br clear="none">>>>>>>> of thousands of them either feeding the input or fed from the <br clear="none">>>>>>>> output of an opamp via a resistor to ground.<br clear="none">>>>>>>><br clear="none">>>>><br clear="none">>>>> _______________________________________________<br clear="none">>>>> Synth-diy mailing list<br clear="none">>>>> <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:Synth-diy@synth-diy.org" target="_blank" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br clear="none">>>>> <a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br clear="none">>>>> Selling or trading? Use <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:marketplace@synth-diy.org" target="_blank" href="mailto:marketplace@synth-diy.org">marketplace@synth-diy.org</a><br clear="none">>>> _______________________________________________<br clear="none">>>> Synth-diy mailing list<br clear="none">>>> <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:Synth-diy@synth-diy.org" target="_blank" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br clear="none">>>> <a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br clear="none">>>> Selling or trading? Use <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:marketplace@synth-diy.org" target="_blank" href="mailto:marketplace@synth-diy.org">marketplace@synth-diy.org</a><br clear="none">>><br clear="none">>><br clear="none">>> _______________________________________________<br clear="none">>> Synth-diy mailing list<br clear="none">>> <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:Synth-diy@synth-diy.org" target="_blank" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br clear="none">>> <a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br clear="none">>> Selling or trading? Use <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:marketplace@synth-diy.org" target="_blank" href="mailto:marketplace@synth-diy.org">marketplace@synth-diy.org</a><br clear="none"><br clear="none"><br clear="none">_______________________________________________<br clear="none">Synth-diy mailing list<br clear="none"><a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:Synth-diy@synth-diy.org" target="_blank" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br clear="none"><a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br clear="none">Selling or trading? Use <a rel="nofollow noopener noreferrer" shape="rect" ymailto="mailto:marketplace@synth-diy.org" target="_blank" href="mailto:marketplace@synth-diy.org">marketplace@synth-diy.org</a><br clear="none"></div></div></div>
</div>
</div></div></div></div></div>
</div>
</div></body></html>