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  • Questions about inputs to Allen MOS-1 T-20 amplifier

    The last time I moved my Allen Model 120 organ away from the wall, I wanted to increase the volume of the Flute family - it is noticeably softer than the Principal stops. Basically I cannot hear any added input from the Gedackt 8’ or Spitzflote 4’ on the Swell when the same note is played with the Principal 8’ on the Great.

    I turned the gain shaft of the T-20 'Flute' amplifier clockwise and was surprised that the HohlFlote 8 on the Great (and Gedackt 8 on the Swell) did not sound louder. So I turned the gain shaft of the T-20 just short of fully clockwise and then just short of fully counter-clockwise only to hear no affect on the Flute's volume at any of those positions. How can I get any sound through the amplifier if the volume control has no effect? (rhetorical question). Before I charge off and assume that the gain pot is faulty, I conducted a few tests but still have some questions about connections to the Flute amplifier.

    Some test results:
    1. With the top of expression petal fully forward/down, I exchanged the Main / Flute outputs from the DAC board to (what I believe might be) a preamplifier board. Sure enough, the Hohlflute 8 (of the Great) was louder than before the exchange and the Principal 8 (of the Great) was softer. In fact, they were about the same volume instead of the previous Principal loud and Flute soft. So I concluded that the Flute amplifier is working but softer than the Main channel. Unfortunately, I cannot adjust the Flute amplifier's volume.

    2. I pulled the RCA connectors from the expression petal inputs of both Main and Flute amplifiers and measured the expression petal’s dark and light resistance. They were consistent with the values presented in the thread https://www.organforum.com/forums/sh...ion+resistance and reasonably close to one other. I used a flashlight for the lighted resistance measurement since I wanted to keep power off the organ during this test. The expression petal lamp does light (dimly) when plugged back into the side of the expression petal assembly and does influence the volume of both channels equally.

    Some questions:
    a) The Allen T-20 amplifier has two RCA input connections. I believe one input comes from the preamplifier (?) board. Where does the other input to the amplifier come from? These inputs are in addition to the expression petal input.

    b) Can I assume that if I do not plug in the expression connector to the amplifier that I will get an un-attenuated input to the amplifier?

    c) Is it possible for the gain pot to be an open circuit and still get a signal to the amplifier (possibly from the second input)?

    I did have a problem with the T-20 gain pot when I first applied powered to the organ after it was dormant for 40 years. The amplifier would make a loud, scratchy noise that disappeared after I moved the pot from it dormant position. I have not exercised the pot since the initial problem more than turning to max and then to min in the above tests.

    I'm a bit hesitant to jump to the ‘obvious’ conclusion that the pot is bad unless I understand where the second input comes from and whether that input by-passes the gain control.

    Let me just add a note that the picture of the amplifier on the bench was taken in Nov of 2016. Currently, the amplifier is inside the organ and being used for practice.
    Attached Files
    Last edited by Al Offt; 06-06-2017, 10:13 AM.
    Alan

    Allen MOS-1, Model 100, Serial AC-440
    purchased in 1972

  • #2
    Finally got dissatisfied enough with the unbalanced volume between Flute and Main channels to pull the organ away from the wall and confirm that each amplifier worked correctly. Before removing an amplifier, I used a signal (sine wave) generator as an input to each amplifier, one at a time, with the same signal level and measured the sound pressure level of the Main (72dB) and Flute (56dB) channels with a (Radio Shack) Sound Level Meter. Concluded that the Flute signal gain was noticeably lower than the Main. This test was run to also verify that speakers were working correctly.

    Before going any further, let me say that my problem was operator error, not anything wrong with assemblies in the organ. What I thought was the Flute amplifier turned out to be the Main amplifier which explains why moving the volume control on the Main amp had no affect on the Flute channel. After the fact, I discovered that both amplifiers are labeled next to their chassis if you move cables and power lines to the side. Quite embarrassing.

    Of course, I did not discover my error until after I had the “faulty” amplifier on the bench downstairs and found that its gain control worked quite well. With the idea that the flute signal might be "leaking" into the other amplifier (quite a stretch BTW), I went back to the organ, pulled down the Principal 8' and pressed a key - nothing. Lifted that tab and selected the Hohleflote 8' to find that it made a sound. That was a surprise and a revelation.

    After this discovery, I returned the amplifier to the organ and adjusted the other amp's gain control to successfully balance both channels. I was not able to find any tuning procedure for a two amplifier, MOS-1 organ on this forum so I tried playing a key with the Principal 8' selected and increasing the volume of the Flute channel until the Spitzflote 4' blended well without sounding as the dominate stop. While playing a chord, I repeated that procedure and added various Principal and Flute harmonic stops to the Principal 8'. As a last step, I added the 8' Hohlflote and was surprised how that stop filled-in the foundation without dominating the composition. Out of curiosity, I measured the individual sound pressure levels of the 8' Principal and 8' Hohlflote and found both about equal (at 70dB). I'm not sure they should be equal since I have seen registrations where flutes are used for softer passages.

    I'm going to live with this balance for a while but did not check the level of the Petal stops, which use the Flute amplifier, so I suspect I may have to make another balancing adjustment at some time in the future.

    And what about the answers to my questions. From what I read since then and what I discovered,
    a) The third input seems to have something to do with muting the amplifier while the organ turns on and off. When I removed that input and turned power off, the speakers made a loud thud.
    b) After measuring the dark resistance of the LDR as many meg ohms, I'd say that the amplifier’s input will not be attenuated by what is effectively an open circuit in the expression circuit (compared to the input impedance of the amp measured as 98K ohms).
    c) Because the gain pot was not defective and because of my amplifier identity error, I'd say the signal would not get though an open gain pot or via other input to the amplifier.
    Alan

    Allen MOS-1, Model 100, Serial AC-440
    purchased in 1972

    Comment


    • #3
      Originally posted by Al Offt View Post
      Before going any further, let me say that my problem was operator error, not anything wrong with assemblies in the organ. What I thought was the Flute amplifier turned out to be the Main amplifier which explains why moving the volume control on the Main amp had no affect on the Flute channel. After the fact, I discovered that both amplifiers are labeled next to their chassis if you move cables and power lines to the side. Quite embarrassing.
      Off topic but apropos:

      Bought mother and father an electric blanket and put it on the bed. Of course, mother insisted on rearranging it a bit. The next morning were they complaining! Father was boiling all night and kept turning his control down, while mother was freezing and kept turning hers up. Yep, mother had put the blanket on the bed upside down so each was controlling the other's side of the bed.

      For reference, the Swell Fagotto 16 is also on the flute channel. You might check how it balances with the other swell reeds.

      Also, there may be an adjustment for bass boost on the flute channel output. This may help balance the bottom end of the flutes.

      - - - Updated - - -

      Oops, I just noticed that this a 120. The voices may be split differently from the earlier Allen digitals, e.g. the Gt Octave 4 may not be on the same channel as the Principal 8. If the organ has frequency separation you will be able to tell by listening to the tuning as to which voice is on which channel.

      From your post I'd say that you have a good ear for achieving the balance you desire.

      Comment


      • #4
        Originally posted by Al Offt View Post
        I measured the individual sound pressure levels of the 8' Principal and 8' Hohlflote and found both about equal (at 70dB). I'm not sure they should be equal since I have seen registrations where flutes are used for softer passages.
        You may also note, that before your experiments, they were in fact set so that the Diapason rank was somewhat louder... ... welcome to the often very frustrating art of voicing very small organs! I've been there. Years ago I got the church I was playing for a monster custom TC-6 and drove myself crazy voicing it. Do you have Allen's literature on the process? It is worth obtaining. Super large organs can afford to have one of the ranks of flutes at, or near, the strength of one of the Diapason ranks... yeah, instruments that large often have more than one Great Diapason. Some even have more than one Swell Diapason.

        You appear to have discovered the limitations of using single notes for voicing guidance, but chords have to be used in different areas of the keyboard to be effective. Even that has its limitations. I find the best way of doing it is by playing passages of pieces I like. I voice so as to get the best interpretation of those passages that is possible. An SPL meter is a nice tool to have. Next time you are in the instrument you might try knocking down the flute level a tad or raising the diapason, and live with that for a couple of weeks. Chances are good that the way your instrument was set up originally, was how it came from the factory. That is likely as good as it gets my friend. About as good as it gets.

        Comment


        • #5
          Thank you both for your comments and suggestions. It helped to focus my thinking and plan my next steps. Before going much farther, I’d like to attach a copy of the organ’s specification so you have an idea what stops are available and where they are located.

          Leisestrum,
          I had a chance to practice and play some simple Chorales (from Peter's Little Organ Book) which have different registration suggestions for each and concur with your comment that I need to play some actual music before I will be pleased with the balance. Yesterday, petal stops, which use the Flute amplifier, sound too loud and the simple five-stop Cornet in the Swell can hold its own against the Principal 8’ and Octave 4’ in the Great. I do like the higher volume of the flutes but I suspect that preference is because their color is more noticeable at a higher listening level.

          You bring up the good point about factory settings. The marks on the gain pot positions were rather ambiguous (round dots on shaft and chassis) and the gain pots rather sensitive. In my initial noise problem investigation in November of 2016, I tried to reset both pots to their original positions but failed since the Flute volume was so low compared to the Principals.

          MarkS,
          I did adjust (lower) the Flute's bass-boost to balance the lower octave of the Great's Hohlflote 8' against its next higher octave. However, when I connected the petals and played the Principal 16' and then the Bourdon 16', I was a bit disappointed with the response of both in the lower two octaves. After that change, I used a photo of the preamplifier to return that control to its factory setting. And I am very glad I did based on the SPL (Sound Pressure Level) measurements I made this morning (more later).

          After absorbing your comments and suggestions, I decided to move the organ away from the wall just enough (8 inches total) to gain access to controls by reaching in from the side. With organ in that position, I can reach the Flute’s volume control and still have the room to play both manuals and petals.

          [Later in the day]
          Once the organ was moved and before I reduced the Flute amplifiers gain, I decided to record some preliminary sound pressure measurements. Each measurement of manual stops was taken while playing a C-E-G-C chord at various octaves on the manuals and various single notes in the petals. Based on my previous perceived observations, I was quite surprised by the results.

          The Great Principal 8’ chords were within +/-1 dB of each other in all five octaves (at 75dB). The Great Hohlflote 8’ was very loud (90dB) in the lowest octave (remember, I reset the bass-boost), mildly loud (80dB) in the next higher octave and within +/- 1dB of the Principal 8’ (75dB) for all the rest. All singly-played C & G petal notes for the Principal 16’ and Bourdon 16’ were roughly 10 dB above the manual stop volume (i.e. ~85dB) and within roughly +/- 1dB of each other except for the lowest Bourdon 16’ C2 which was soft (66dB). The Bourdon 16’ G above the lowest C had recovered to 85dB. I have included the data as an attachment if you are interested.

          From that point, lowering the Flute amplifier by about -4dB (-3dB intended) produced the expected results and was rather anticlimactic: the manual Flutes and all petals dropped ~4dB in volume from their previous readings while the Principal 8’ remained at the same volume. And the Swell Fagotto 16 did blend well with the rest of the reed chorus. Now I get a chance to see if I like the results over the next couple of days.

          [Conclusion]
          Except for the lower end of the Great’s Hohlflote 8’ and Petal’s Bourdon 16’, I am extremely pleased with relatively uniform volume of both families over all octaves. However, the real test will be whether I like the way it sounds. At least now, controls are within reach and I can make known (like -3dB) changes to the Flute amplifier’s gain and then play a few pieces over a couple of days. If I don’t like the results, I know how much to increase the volume to return to the last-good setting.

          P.S Mark, I liked your story - it certainly rang a bell.
          Attached Files
          Last edited by Al Offt; 06-16-2017, 05:26 PM.
          Alan

          Allen MOS-1, Model 100, Serial AC-440
          purchased in 1972

          Comment

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