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Author Topic: high end frequency response  (Read 9611 times)

Offline Jayeltex

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high end frequency response
« on: November 24, 2010, 09:07:38 AM »
I am running a rewired RS1500 (heads directly out) through an Eros tape head preamp

Overall I am finding a lack of smooth open high end frequency response and not much "airy presence" in the sound which I believe should be there. In fact if anything I find the bottom end a little thick and heavy.

Could it be my Line preamp (an EAR 834L) ?

Any suggestions ?

Offline docb

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Re: high end frequency response
« Reply #1 on: November 24, 2010, 10:53:44 AM »
Have you performed a head alignment, and do you have any frequency response numbers to share? The measured output levels at 10kHz and 15kHz relative to the level at 1kHz would help us to help you. You can plug the outputs of the Eros into the line inputs of the Technics and meter that line input. That way you can monitor the frequency response on the VU meters as you run a tone tape.

Also you might want to double check that the playback EQ setting is correct for the tape you are listening to.

As far as the bottom end response goes there is usually a bit of a head bump in the mid bass with stock heads
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline Ki Choi

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Re: high end frequency response
« Reply #2 on: November 24, 2010, 10:58:52 AM »
Of course the best way to determine the performance of your set up is to run a known calibration tape such as the Tape Project IEC cal tape or the MRL cal tapes and measure its frequency responses at multiple frequency points.

Have you checked the condition of the repro head (including making sure it is clean...)?

Based on my experience, one of the first sound impressions from a direct-head-wiring to an outboard pre-amp was more "air," greater detail with more information, and almost too much high frequency - compared to less than ideal factory repro electronics in most of the pro-sumer machines that are designed more for flexibility than for pure audio performance, IMHO...
Ki Choi

Offline ironbut

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Re: high end frequency response
« Reply #3 on: November 24, 2010, 01:37:46 PM »
Hi Jay (?),

Are your impressions in comparison to the way the machine sounded before the rewire? Also, knowing what tape are you using for this evaluation will help us help you (is it a Tape Project tape, old 1/2 track/1/4 track commercial release or something you recorded?).

How did you rewire your heads? Did you use the stock wiring and just re-route it or did you use some new wire?
When I re-wired my heads with Cardas 24 AWG shielded cable, it took a bit of burn in time before the highs opened up and the bass tightened up. If the Eros is new, it could also need burn in.
What I'm talking about here are fairly subtle changes. The twinkle might seem a little distant and the bass sounds a little flabby.

Doc and Ki are very are right about the need to use a calibration tape. The heads should be clean, with a nice even surface and adjusted correctly to get the most out of the machine.
steve koto
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Offline Jayeltex

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Re: high end frequency response
« Reply #4 on: November 24, 2010, 06:20:02 PM »
Dear Doc, Ki and Ironbut

Many thanks for your quick responses and your suggestions.

The heads are very clean, having the 1500 for about 3 weeks, the heads have been cleaned at least 3 times (weekly).

With the exception of a few quick trials using some older tape to make sure that everything was working, the only tapes that has been payed
are TP tapes (The Band and The Staple Singers...that's all that were available..I'm waiting for Eileen to call me when the balance of my selections come in)

I did visit my local electronic guru (he is an ex Studer technician) and had him do a careful head alignment on his scope. In fact my first inclination was that the heads were out of alignment. Any small misalignment has now been corrected using his (TEAC company issued) test tape. According to the scope, we have a nice clean flat 20-20 response.

As for the direct out head rewiring, I presume the technican just re-routed directly out to the RCA's.

Could it be a bad tube ?  Is it possible that something happened to the Eros in transit (Eileen told me that Sean checked it carefully and it sounded great before shipping) ?  Is it time for new heads (I think they look ok)...before I have to send the headblock off to Fluxmagnetics (for new heads) I was hoping to find a less expensive solution.

I'm grateful for any other suggestions..

Thank you
jay




Offline Jayeltex

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Re: high end frequency response
« Reply #5 on: November 24, 2010, 08:02:11 PM »
I just spoke with Tom McCartney, the technician that did the recent adjustment. He told me that the his readings were off
2db @ 10kHz and 3db @ 15 kHz.

Please let me know if this information is helpful.


Thanks
jay

Offline docb

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Re: high end frequency response
« Reply #6 on: November 24, 2010, 09:34:41 PM »
Were those -2dB at 10kHz and -3dB at 15kHz readings through the Eros, or through the stock electronics? Were they exactly the same on both channels?

If the measurement was through the stock electronics, and/or if the rolloff is different between channels the playback head is probably worn and needs, at the very least, a relap.

If the rolloff was measured through the Eros, the next question is how long and what kind of cables are between the Technics and the Eros? Some cables can really roll off the top end, so even if the measurements were taken with the Eros it could still be a worn head or it could possibly be incompatible cables*.

The Eros was tested on a known good deck here and the response was flat, so it's reasonably unlikely that anything other than the wrong EQ switch setting would make the Eros responsible for the rolloff. You could send it back and we could test it, but without the actual deck and cables you are using it would be impossible to say exactly what the issue is.

* I will note here that "incompatability" has nothing to do with the price or reputation of the cable used. It all boils down to cable capacitance, which varies with brand and model, and always increases with length. In general you want the lowest capacitance possible. Short cables (like 1/2M) are always a good bet, and a cable with something less than 20pF/ft capacitance is a good bet.
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline ironbut

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Re: high end frequency response
« Reply #7 on: November 25, 2010, 01:08:38 AM »
I'm still trying to get a handle on what you're hearing. Specifically, how severe is the roll off and the bass bloat is.

If it's somewhat subtle and you have to listen closely to hear these, it could be cables like Doc was saying or if it's obvious and striking (dull and boomy) it could be something basic.
For instance, I see that you used to use tape but a few things have changed. All current production tape is backcoated now. Because of this, it looks a little different. The back is black with a matte finish while the oxide side that contacts the heads is brown and shiny.
Another thing particular to the RS1500 is that there are two playback heads a 1/4 track and 1/2 track one. There's a switch on the headblock that toggles between the two heads. Unless the headblock has been altered or exchanged, when the switch is to the right, the 1/2 track head is connected which is what you want for the Tape Project tapes. This switch can sometimes get dirty but switching it back and forth a few times will usually cure this.
Also, be sure that you've threaded the tape correctly. There's an illustration on the top of the machine.
steve koto
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Offline Jayeltex

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Re: high end frequency response
« Reply #8 on: November 25, 2010, 01:27:13 PM »
Hi Doc and Ironbut

Thank you again for your continued responses and I will try to answer all of your questions

Confirmed that the response readings coming out of the Eros were the same from both channels.
I am using Chord (Cobra 2) 1 meter interconnects (not very expensive)  from the RS1500 to the Eros and from the Eros into my Line stage preamp and according to Tom McCartney (my technician and electronic guru) he figures they are about 15pF/ft
The Eros is always set to IEC
The tapes are definitely threaded correctly (also I know the difference sonically between the right and wrong side of the tape...huge difference)

I am not saying that it sounds like the wrong side of the tape is running...it just doesn't sound "open" and "airy".. The top end is
not clear...crisp. I get more response from vinyl and from CD. Somehow I just know (and can hear) that there is "lots" more on the tape that is not coming
forth.
I have asked Tom to see about getting me a brand new good quality play head but I just keep hoping that maybe it's something that I am missing.
Is it possible that I need to get a small eq built into the Eros to match the play heads output from my RS1500 (kind of like the elevator on my Graham Slee phono stage) ??

I'm willing to try anything at this point. Perhaps I could have Tom call Doc ?

Look forward to your further comments

Thanks again
jay




Offline docb

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Re: high end frequency response
« Reply #9 on: November 25, 2010, 01:57:59 PM »
The simplest place to start is to put a different pair of interconnects between the deck and the Eros and see what happens. We have designed all of our tape head preamps with Technics decks and heads in mind, and based on our past results there should not be a need for any compensation in the preamp itself. On some other deck and cable combos we have replaced the input grid resistor with a 250K ohm trimmer pot to allow for adjustable head damping. Typically that happens when we see a peak, not a rolloff. I have not seen this necessary with a stock Technics head in good condition. If the cable isn't the problem and the head is worn a head relap is only about $85, a lot cheaper than a new head or building some custom eq.

If Tom wants to call I will be back in the office on Monday. But trying some different cables is the very best and easiest first step. We have this one pair of cheap cables that Shawn occasionally forgets I have banned from the test bench (probably because I never leave him another pair of dedicated "bench cables"), that always show that kind of rolloff when we do an alignment using them.
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline Jayeltex

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Re: high end frequency response
« Reply #10 on: November 26, 2010, 10:51:48 PM »
Hi Doc

Thanks very much for your suggestions and for your kind offer for Tom to call you. Changing cables is an easy test and I will try to do that over the weekend to
see what happens.... but based on what I am hearing, I am afraid that it might be the head and have asked Tom to see about picking up a new one for me to try.

I have high hopes and anticipations on what the sound quality should yield and will keep you updated on our progress.

hope you had a nice Thanksgiving !

Jay

Offline Jayeltex

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Re: high end frequency response
« Reply #11 on: November 30, 2010, 05:02:53 PM »
Hey Doc


Well we have tried just about everything (excluding a new play back head) and we are still in the same place. I would be grateful for a few minutes of your time on the phone.

Please let me know when it would be convenient to call and at which phone number.

Thanks
jay

Offline docb

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Re: high end frequency response
« Reply #12 on: November 30, 2010, 06:34:06 PM »
I should be in the office tomorrow by about 10:30 a.m. PT.
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline Jayeltex

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Re: high end frequency response
« Reply #13 on: November 30, 2010, 06:35:48 PM »
Great, thanks. Look forward to chatting with you


Offline docb

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Re: high end frequency response
« Reply #14 on: November 30, 2010, 07:46:15 PM »
Oops forgot the phone number - 206-451-4275
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project