From D.S.Walker@bradford.ac.uk Mon Jan 18 15:12:35 1993 From: D.S.Walker@bradford.ac.uk (Simeon Walker) Newsgroups: comp.sys.acorn Subject: Re: Arc sound - impedance matching Keywords: audio, impedance Date: 14 Mar 92 14:26:17 GMT Organization: University of Bradford, UK Originator: 91909178@ccw403 Nntp-Posting-Host: ccw403 In article <1992Mar13.013055.11591@citec.oz.au> sgccreg@citec.oz.au (Rob Geraghty) writes: >Do we have any engineers in comp.sys.acorn? > >I have my Arc connected to the auxilliary inputs on the stereo. >The Arc sound output is designed to drive 30 Ohm headphones. >The aux in is 47KOhm. >(no lectures about the difference between input and output impedance please) > >Does anyone have a circuit diagram to impedance match this connection? > I used to drive the AUX input on my Creek amp from the Arc headphone output and had no problem. As for impedance matching, transmission lines were never a strong point of mine. >The high end of the Arc sound seems to get lost in the translation - judging >by the difference in the sound coming from the Arc speaker compared to the >sound from the stereo. > I think the problem here is that since the speaker in the Arc is so poor at the low end it gives the impression of having a better response than your stereo at the high end. Also, I do know that the headphone output first passes through a low-pass filter to remove any unwanted high frequency components caused by the digital to analogue conversion. Unfortunately the filter may be a bit severe hence the dull sound. I don't know if the sound for the internal speaker is passed through this filter. It is, however, possible to bypass this filter. Inside the Arc (400 series at least) there is a 2x5 array of pins near the headphone socket which provides an unfiltered audio output. I now feed this signal to my amp which gives a much better sound, most of the time. Sometimes a bit of high frequency noise is noticable. I can find out the exact pin numbers if you want. >Also I suspect the levels need matching as well. I think the Arc sound is >getting distorted by the amp input stage - this makes a lot of sense since >the voltage to drive a pair of headphones would be a *lot* higher than >standard phono line-level. I just hope the Arc isn't likely to produce >anything that would damage the inputs on the amp! > I'm confused now, are you using a sensitive phono input or a normal line level input. I wouldn't even think about using any phono input. You should be able to use a tape,tuner,CD or Aux input with no problems. That is unless your amp has very low overload margins on those inputs. >Ideas anyone? (there may be an article from a mag I've missed somewhere...) > >TIA > >Rob G -- Sim. ----------------------------------------------------------- Simeon Walker, ----------------------------------------- Department of Electrical Engineering, D.S.Walker@bradford.ac.uk -------------------- University of Bradford, England. From julian@bridge.welly.gen.nz Mon Jan 18 15:12:48 1993 From: julian@bridge.welly.gen.nz (Julian Wright) Newsgroups: comp.sys.acorn Subject: Re: Arc sound - impedance matching Keywords: audio, impedance Date: Sun, 15 Mar 1992 00:47:06 +1300 Reply-To: wright_j@kosmos.wcc.govt.nz (Julian Wright) Organization: USS Enterprise, NCC 1701a sgccreg@citec.oz.au (Rob Geraghty) writes: > Do we have any engineers in comp.sys.acorn? Well I'm not an engineer, so I accept no responsibility for whatever happens to you or your machine if you do the following (I imagine it will void your warrantee too, if you still have one), but... The Problem: > I have my Arc connected to the auxilliary inputs on the stereo. The Symptom: > The high end of the Arc sound seems to get lost in the translation The Cause: Low Pass filter present. The Solution: Tap sound output before it reaches the Low Pass filter. The Method: - Open your 300, 400(/1), 500, or 5000 series machine up, and find a 5x2 pin connector somewhere in the video circuitry. There's only one, and on the 310's I have seen it is near the back-right corner of the circuit board. On the 5000 it is around the back-center of the board I think (it's the only such connector that I can see on the board). It looks something like: 9 . . . . . . . . 1 . . - All the pins on the right (even numbered ones) are ground. Pin 1 is LEFT sound output and pin 9 is RIGHT sound output. I have no proof but guess that pins 3, 5, and 7 are raw R, G, and B video signals respectively. - Attach a 3.5" socket to a short length of ribbon cable and clamp a 5x2 IDC plug on the other end. (Much better than soldering wires directly to the pins). I use pin 8 for ground but in theory any even numbered one will do) I assume you know the layout of a 3.5" socket 'cos I can't for the life of me remember it at the moment! It's in the User Guide in one of the appendices anyway... - Dangle the socket out the back through one of the cracks (if there are any...) or better still, drill a new hole in the bottom back panel right next to the current headphone socket hole and mount the new 3.5" socket in it. You now have TWO output sockets, one filtered, one unfiltered. - Attach your stereo to the new socket, which works equally well as a headphone socket or a line out. I find that without the filter STMs sound much better, but low grade samples sound a lot more scratchy. This is understandable, as STMs are played at 29KHz, while low grade samples are less than 20KHz, usually 13 or 10KHz. Apparently the filter is there to make the sound from the internal speaker sound reasonable. Shame it also affects the standard external connector, otherwise all this hacking around wouldn't be necessary. This "upgrade" is also possible on the A3000, but you have to hook into a couple of resister legs (I don't know which ones sorry) as the 3000 doesn't have the 5x2 connector on the circuit board. Cheers, Julian. -- ----------------------------| RO2:SYS 68,59243844 |-------------------------- wright_j@kosmos.wcc.govt.nz |---------------------| "I'm sig of sicknatures!" ----------------------------| RO3:SYS 68,60816742 |-------------------------- --