It was long my Yamaha NS-10 monitors had both tweeters dead, I love these speakers so much that I was costantly thinking on fixing them, but didn’t want to spend a fortune. Unfortunately, the blown tweeter is a very common issue with these monitors and as the NS-10 remains an indispensable tool for lots of people, the original tweeters are still in high demand. As the NS-10 speakers are no more in production that is long already, finding parts was getting pretty hard and the originals, reached very high prices. This is the reason that forced me years ago, to buy a pair of Yamaha MSP-5, monitors that I love very much, but that are not “my” NS10s.
Last week, with great surprise I discovered that Avantone Pro, the company of those classic tiny monitors, are selling identical copies of the original NS-10 speakers and at a really attractive price. After reading all I could and watching a few videos, I decided this was the best way to go and wanted to give a try to these replacement tweeters. I bought mine through a big online shop at 122 euros each, really a fraction of what I could have spent for NOS Yamaha NS-10 tweeters.
As promised the replacement is a drop in, there’s no need of adapting anything, the shape of the replacement speaker is identical to the original. All is as simple as soldering the speaker cables to the new tweeters and screw the tweeter back into the monitor.
I’m surprised at looking how identical to the original these tweeters are, all details are exactly the same, only the ring around the tweeter dome, has a slightly lighter grey colour. Is it this the difference that allows Avantone to clone the Yamaha speakers, without copyright infringement repercussions? Sure you understand I’m just kidding, I’m sure if they can freely sell a so exact copy, these details have been cleared already.
Original Yamaha tweeter on the left
After the installation and the promising sound test, I’ve been mixing for several hours, completely forgetting I was doing so on the new tweeters, perfect prove that I was feeling absolutely confortable with their sound!
Before finding this DIY kit on the AML online shop, I didn’t know anything about Fairchild equalizers. I knew this manufacturer was famous for the 670 limiters that I had used many times in different software reissues and that was the same Fairchild factory, building those fat Flying Boxcar planes that used to impress me very much, roaring low over my head while a kid.
Pultecs as rain
As I had no tube EQs in the studio, my initial idea was building a couple of classic Pultec equalizers clones. Sure there is no need to explain more, my typical readers can possibly know more than me about. Searching for the best way to go, I remained surprised to see how many clones or Pultec inspired equalizers exist today in the market, in the DIY or finished product form. Even big manufacturers like Behringer, recently introduced their Pultec, under their Klark Teknik brand, at an insanely cheap price.
Looking on the AML web for info on their Pultec kits, I totally fell in love for their Fairchild 627 reissue, instead. When I crave for new gear, I try to stay tight to my technical side and using logic, considering what is really needed. This brings me most of the time to fix, mod and recycle old gear still in my hands, instead of buying something brand new. OK, after decades soldering and fixing things, I can consider myself a bit a tech too, but I’m mainly a musician, then deeply attracted by fantasy and details that can look futile, but are extremely fascinating to the non-logic (illogic?) part of the mind! That green glowing “Magic Eye” on the front panel was too intriguing and it was calling for my eyes!
I had seen it already, so far in time but still, it was so intensely fixed in my memory! Like coming out from a Sci-Fi movie, the feeling was a mix of curiosity and fear, so green and scary to only possibly being an alien artifact and sure able to promptly take control of my mind! :D
The “Magic Eye”, the VU meter grandfather
Before the 1960s, needle VU meters were extremely expensive, high tech precision tools mainly for scientific equipment, telephone and military use. VUs started becoming a practicable form of measuring an audio signal, only after mass production started in Japan and LEDs made the application on consumer products, very cheap. Electron ray indicator tubes were the way to go, to have a reliable visual representation of the presence and amplitude of an audio signal. First implemented on radios to make it easier tuning to a station, they can be found on vintage tape recorders and televisions too. The magic eye is a miniature cathode ray vacuum tube, the same as the screen of old televisions, plus an amplification stage, like in usual tubes. While the rest of the “screen” (anode) is illuminated by the electrons strikes, a part (electrode) is kept dark until a signal is detected and the flow of electrons is allowed to run in that area too.
Building the AML Fairchild 627 EQ
Once the kit arrived the connection with aircrafts strangely was evident again, this time in the form of a deja vu: The PCBs were cleverly cut in the resin board, in a form it resembled me the plastic aircraft model kits so popular in my youth, where you snap off the part you need from the mainboard, through pre-cut junctions.
AML ez-627 PCBs
My previous DIY builds never started from complete kits, all I usually had were a couple of PCBs and a BOM (Bill Of Material) list. Often, finding the right components to buy and where can become a boring and long process. This time there was nothing to worry about, only buying the valves that weren’t included in the kit. These are pretty common tubes available easily, what I couldn’t find were matched pairs, as I would like to have installed, at the beginning.
AML ez-627 unboxing
The quality of the boards is excellent and all components locations are labeled carefully. Same with the electronics, included in the kit are quality parts from manufacturers like Wima and Vishay. The transformers are Carnhill, the same I installed years ago in my DIY Neve 2254/33609 hybrid compressor (hopefully a good theme for a future post).
Assembling the 627
Arrived at the end of the project, the two Russian 6E5C/6E5S (C is an S in Cyrillic) hadn’t arrived yet, they were lost somewhere in the north of Spain. Calling the post office they told me my “Magic Eyes” had been held by customs (?), I can imagine the officers looking at my alien-ware, guessing what the hell are those for…
Building the tower for the Magic Eye, like somewhere in Mordor…
How does the 627 sound?
Even if the choice of building this processor might having being influenced not only by technically related reasons, I’m very satisfied with the results. All I passed through my units, in a couple of days they are alive, sounds very full and warm. I only tested my new pair on bass and guitars while mixing a new song, yesterday. As frequency action, its higher band goes up to 10kHz, below the 16kHz a Pultec EQP-1A (my initial choice), can work. The 627 is not too useful for brightening the higher part of the spectrum or shimmering, but very nicely working on lower frequencies. Testing it on bass, the tone I obtained was super fat and the action way more dramatic than I expected, in seconds I had a huge synth bass ready. I had to bounce the track to free the unit for processing some guitars, but I forced myself to limit the test to them only, or I would have started passing the whole song track by track, through the 627s! :D
I hope to make a follow up to this post, with the help of my OSO http://www.originalsoundsonly.com squad, with some audio tests, soon.
There’s no doubt that the Neumann U47 is one of the most respected valve microphones ever made, been used to record the best vocals in the history of music. For this reason, lots of copies or variants have been built in the last 50 years, up to been recreated recently, even from one of its original manufacturers.
The hardest aspect to reproduce has always been the behaviour of the valve used, the Telefunken VF14, a military radio tube discontinued in the 60s (this caused Neumann to stop the production of this model) and very hard to find, until Telefunken reissued it under the name VF14K, a few years ago.
ioaudio MK-47 kit 01
I’m not a microphone expert, I only had built a pair of AKG C12 “clones” in the past, based on the Apex460 and made a few repairs of old micros, but absolutely wanted “that” sound and a tube microphone sounding different than my pair of DIY C12.
I found this project a few years ago on GroupDIY, while it still was called prodigy-pro.com and I liked the idea of using a pair of 6028 tubes to reproduce the VF14 and wanted to give it a try. Building a PSU and finding a microphone body hasn’t been really a problem, as in the same moment other builders had offers of kits for the ’47.
I remember opening the box, the kit’s more curious parts were the Soviet condenser with Cyrillic markings on it and the specially tailored transformer, reproducing the original Bv.08 trafo.
ioaudio MK-47 kit 02
I don’t remember the MK47 building to be difficult at all, all was explained very well in the MK47 Build Support thread of the groupDIY thread:
Max Kircher’s ioaudio is still building kits for vintage microphones, their actual kit is an upgrade to transform a U87 into a tube U67, something I would like to build, one day for my 87.
Thiersch STW7 Red capsule, front
As capsule I chose the Thiersch Elektroakustik, a perfect reproduction of the M7 capsule, with the membrane in two different flavors, PVC or PET (mine).
DIY U47 PSU
More tricky was building the power source, but I’ve been lucky to find a suitable pcb in Prodigy’s “White Market”.
As body, I used the Equinox Systems copy of the U47 enclosure, very nicely done.
This remains up to today my preferred microphone in the studio, I use it mainly on voice.
The MXR M-113 Digital Delay has been one of the most popular first generation audio delays, as it had a good quality but was more affordable than the previous processors. Introduced in ’76 at around 1000 USD, it was still expensive, but a lot less than the other digital delays available in the market.
MXR was specialized in guitar pedals, this delay was instead offered in a 19″ rack unit and, same as the other effects of the professional rack series, (Flanger/Doubler, Graphic EQ, Pitch Transposer) became very popular and used by famous artists such as David Gilmour, Frank Zappa, Roger Waters, Brian May and Mike Rutherford.
Modular
MXR memory module
To contain costs, a modular architecture was chosen. Optional memory boards were inserted in slots, to increase the delay time, but these were pretty expensive, costing almost like a complete unit.
MXR Digital Delay Internal Cards
Digital noise..well not only that! :D
Unfortunately the M-113 Digital Delay, same as its collegues of the same period, is pretty noisy and not only referring to the actual standards. Apart digital noises always present, these processors have a marcated tendency to generate audio hum.
The Model 113, Digital Delay has a maximum bandwidth of 20KHz, but this decreases with longer delays, down to 2.5KHz. The longer the time between repetitions, the tighter the passing frequency band, being reduced through antialiasing filters (HPF and LPF) the more the delay time is increased.
MXR Digital Delay Test Drive
As there are several video examples already of this delay tested on guitar, I thought it could have been a good idea to try it, using some unusual but hopefully interesting sounds.
To record, I used a dbxDI box and the signal sent to a Universal Audio610 tube preamp, even taking all possible counter-measures, still a bit of hum is audible.
On the examples below, I feeded as sound source, a sweep from the filters in feedback of an old Korg MS-50 synthesizer, some piano chords I had recorded from a real piano and a couple of seconds of an oldschool reggae classic.
MXR Digital Delay, Test-01, (MS-50 Filter Sweep)
01) Dry sound, straight from the MS-50.
02) Delay Range 160ms, Delay Time x2, 50% mix and maximum regen (feedback)
03) Delay Range 160ms, Delay Time 0, 50% mix and maximum regen (feedback)
04) Delay Range 160ms, Delay Time 1/2, 50% mix and maximum regen (feedback)
05) Delay Range 160ms, Delay Time Sweep 01 (moving the Delay Time knob)
06) Delay Range 160ms, Delay Time Sweep 02 (moving the Delay Time knob)
MXR Digital Delay, Test-02 (Piano)
I chose these piano chords I recorded as, having long decays, are ideal to test the effect of short repetitions, when modulated. The effects space from phasing to complete madness. The last one is a strong modulation at the higher band of frequency of the sweep, reminding the effect of a ring modulation.
1) Dry Piano
2) Delay Range 1.3ms, 50% mix 3) Delay Range 10ms, 50% mix
4) Delay Range160ms, 50% + Mod
5) Max Frequency Mod
MXR Digital Delay, Test-03 (Rhythmic Loop)
On the following examples, let’s try something that really impressed me first time I played with a Powertran (An article on it, is coming soon). On the MXR the feedback has a faster decay and there’s not the “freeze” function, but the effect it is interesting, too. Once the delay is at tempo with a rhythmic pattern, having adjusted the decay time knob, all Delay Range buttons create a different version of the pattern, but on different speeds. Being the buttons based on a double time sequence, all these are compatible in tempo. Let’s experiment with a small classic reggae riddim loop:
1) Dry loop
2) Loop effected as sort of Beat Repeater
MXR Digital Delay, Test-04 (MS-50 patch)
Last example, this time using a more complex sound from the MS-50.
1) Dry MS-50 sound
2) MXR MS – 50%MIX, 75% Feedback
3) MXR MS – 50%MIX, 75% Feedback + Mod
4) MXR MS – 50%MIX, 75% Feedback + Fast Mod
**Updated February 27, 2026**
Refreshed the English for better flow and readability. Core ideas unchanged — still celebrating the "procrastinated" sound of vintage echoes and delays. Planning audio updates and continuations (RE-201 Perry dub, SDE-3000 Mad Professor style) soon. OSO certified as always! 🎶
Echo, unlucky nymph
Since ancient times, places destined for public speech or shows were chosen based on their acoustical characteristics. A powerful voice heard near a sharp rock wall could impress the audience and add a magical ingredient, as if the forces of nature were happy to help the venue by amplifying the desired message.
Nymph Echo on a Roman painting
The Echo was so important that it had a dedicated character in Greek/Roman mythology:
Echo, a mountain nymph, helped Jupiter distract his wife Hera with endless discussions while the god was cheating on her with other nymphs. Realizing the trick, Hera cursed Echo, removing her ability to speak. The only sounds she could make were the endings of the words she heard. As usual in ancient myths, the story ends in tragedy, with Echo dead but her beautiful repetitive voice forced to live on forever — only in the most remote and inaccessible places.
Natural echoes or reverbs from special rock formations or the reflective shapes of caverns were used to create dramatic effects on human speech, chants (and minds) for ritualistic or entertainment purposes.
The Art of Procrastination
Builders could, over time, recreate nature’s vast spaces through architecture. The possibility of impressing the public with special effects slowly moved from remote areas to cities. Acoustically shaped amphitheaters, and later closed spaces like theatres and religious buildings, were constructed for this purpose. Music began to be played in dedicated “chambers” or “halls”, with reflective walls and high ceilings, to achieve long reverberation times and decays.
At the beginning, recording studios captured the space where musicians played, with microphones pointed toward ceilings or walls to pick up the returning delayed sound, which was then mixed with the original.
Later, rooms were dedicated exclusively to reverberation. A sound from the control room was sent to an amplifier in an empty room or chamber, completely bare and lined with reflective materials. One or more microphones captured the effect, which was mixed back into the rest of the sounds on the mixer. To add color or create special effects, thin metal plates or springs were also made to resonate in the room and recorded.
Plates
By the end of the 1950s, technology had advanced to the point where the reverberation process could be standardized. Every major studio had a plate reverb like the EMT 140, or a simpler DIY version. The effect was achieved using a large reflecting surface — in this case, a metal foil held in tension by springs. A speaker inside the unit vibrated the plate, and a pickup captured the signal, which was then amplified through a tube preamp. A mechanical damper reduced the decay, and later versions used two pickups in different positions to create a stereo signal.
The effect is warm and beautiful, but unfortunately the technology required large spaces — the plates were often more than 1 meter long — and these reverbs were very heavy and expensive.
The effect was obtained through the use of a big reflecting surface, in this case a metal foil put in tension through springs. A speaker inside the unit, puts the plate in resonation and a pickup captures the signal then amplified, through a tube preamp. There was a mechanical damper, to reduce the decay and later two pickups, placed in different places of the box, to obtain a stereo signal.
Spring Reverb
Lee “Scratch” Perry and his Grampian spring reverb (Black Ark Studios)
A lower-quality delayed signal could be obtained using the same process but with springs in tension instead of plates. The idea, commercially developed by organ pioneer Laurens Hammond, was much less bulky than plates and even portable — to the point that it became standard on electric guitar amplifiers. Spring reverbs give a particular vintage flavour to guitars, drums, and percussive instruments — really great for old funk or reggae.
Tape Echoes
With advancing technology, magnetic surfaces in the form of tapes (Echoplex) or disks/drums (Binson/Meazzi) were used to record and reproduce delayed signals on dedicated processors.Roland RE501, RE-201, RE-150 Tape Echoes (maxproaudio)The tape loop was the first form of reproducing sound with delay. Two tape recorders were used before dedicated machines existed: one to record and the other for playback. The longer the distance between the record and repro heads, the longer the delay time.
Roland RE501, RE-201, RE-150 Tape Echoes (maxproaudio)
The tape loop has been the first form of reproducing a sound with a delay. Two tape recorders where used before dedicated machine were born, one to record and the other for the playback. Delay time increased, the furthest the second recorder was placed from the repro machine.
One of the most popular tape echo units has been the Roland RE-201 Space Echo, still highly sought after by dub producers today at pretty high prices. Its tape was left unrolled in a transparent compartment inside the unit. Raising the top and watching the tape move in an apparently chaotic way, listening to the smooth mechanical noise while smelling the characteristic chemical odour, remains a very unique experience.
Some special effects obtained with tape echoes were quite popular in the 60s and 70s, especially for sci-fi and dub. I recorded a few examples with my RE-201, with the “Intensity” (feedback) at maximum — conditions under which the Space Echo starts auto-oscillating, feeding its own noise.
The effects obtained are warm and immediately bring you back to the past. The first track, called RE-201 Slam, is obtained by slapping the unit forcefully; the shock is captured by the internal spring reverb. The third track starts dry and adds effects from the second part. Tracks 2 and 4 are sweeps, moving the “Repeat Rate” knob.
Many virtual tape delay plugins today are based on the RE-201 Space Echo, the most popular in the RE series.
According to Wikipedia, a bucket-brigade device (BBD) is a discrete-time analogue delay line, and an analogue delay line is a network of electrical components connected in cascade, where each element creates a time difference.
A series of capacitors created the delay. Summing a large number of them could produce the desired long echo.
Analog delays cannot reproduce the same sound for long — each repetition gets dirtier — and this, from the perspective of that era, was seen as something to improve. They are still very popular and produced for guitarists.
Digital Delay Lines
Digital delays can repeat the same sound while maintaining the quality of the original, just like a sampler, only when the sample rate and bit resolution are high enough.The first commercial machines had very low resolution, so the samples were noisy and had limited bandwidth. This is the characteristic that, in my opinion, makes them attractive and useful even today, when clean sound is no longer the priority.
At the beginning, these machines were only within reach of professional studios or wealthy producers. Then Electronics & Music Maker Magazine published a series of articles with schematics and instructions for a DIY delay, making this new technology affordable and accessible to a wider public.
Being a lover of dub and vintage equipment, I’ve been lucky to own a few of the classic delay devices, bought when prices were extremely low because nobody at that time needed a bulky, noisy delay-only machine.
Vintage delay lines have an other interesting aspect.
Powertran Time Delay switches
Their buttons always select multiples of the original tempo. If the delay is in sync with the track tempo, pushing different switches gives a very fashionable beat repeater effect today, where, for example, a loop speeds up or slows down but always stays in sync with the song. Some machines, like the Powertran, had a freeze button, so the sample repeated without decaying — just like a sampler.
Effects obtained apart the Echo
A delay line can create a series of effects not strictly limited to repetitions of samples. By modulating the processed signal and blending it with the original, a range of interesting effects can be obtained.
Phasing
When the same signal is combined with its delayed copy, it’s like passing it through a filter. The resulting comb filter, if modulated by changing the delay time via a low-frequency oscillator (LFO), becomes the pleasant effect called phasing.
Flanging
Same as phasing, but with a longer delay (around 10ms). Similar to chorus but more dramatic, with a marked sci-fi character.
Chorus
Even more delay and slow modulation, possibly with dry and wet signals on opposite sides of the stereo field, gives the classic chorus effect that doesn’t need much more explanation.
ADT
Automatic Double Tracking,
Automatic Double Tracking was very popular in the 80s. The time delay is kept just below the perception of individual repetitions and is used especially on vocals to create the illusion of doubling without recording two different voices.
On the next posts, we will test drive three of the old-school digital delays still in my hands: MXR Digital Delay, Powertran Digital Delay Line, and the Roland SDE-3000
